Sorry for the vague title, I just couldn't find a better way to phrase it. Basically, this thread is for discussing what new content/mechanics you wish could be added to the game, but can't due to a variety of reasons. 

Disclaimer: This thread is not to rant about WotC's new changes for inclusivity. Do not post in this thread ranting about lineages, "cancel culture", "twitter mobs", or anything else related to that. You have been warned. All posts that violate this rule will be reported for both being Off-Topic and Trolling (as I have very clearly warned you, and any post that violates is will just be trying to derail and devolve the discussion). Go to a different thread to discuss the lineages and voice your opinion on them. 

Here are some examples: 

Anyone that has read the Beyonders trilogy by Brandon Mull will likely recognize these races; the Amar Kabal and Displacers (the series also has the Drinlings, but frankly, they're the most bland of the three races this series creates, and not really worth mentioning in this thread as they can fairly easily be made as a race in 5e). 

• The Amar Kabal are a race of "basically elves" that are great warriors with long hair that can live for practically forever, though they achieve this differently from Tolkienish elves. The Amar Kabal age at around the same rate as humans, but have fleshy "seeds" that grow from the back of their neck that will fall off of their body if they die, storing all of their memories inside of the seed. If the seed is later planted in hospitable soil, the Amar Kabal will regrow within a few months at the same age that they were when they first died (typically around 20).  The reason this race could not exist in 5e is mainly because of the issue of letting a player live forever as long as the seed isn't destroyed, as well as the thematic overlap with elves. Although I love this race in the Beyonders series and their lore, and I wish there was some way to translate them to D&D 5e, there is alas no way to do it without the race causing all sorts of problems. 
• The Displacers of Lyrian are a race that spies for an evil wizard, and they're the best spies around due to their unique . . . physiology. If you cut off a part of a Displacer's body, that Displacer will be able to move the body part as if it were still connected to its body. Basically, their detached body parts are connected by "portals" called "displacement fields" that allow blood, oxygen, and other necessities to flow between the limbs without spilling out of the holes in their body and causing them to bleed to death. Body parts that are burnt, dissolved by acid, cut/ground up into small enough pieces will act as a human would when losing that body part, losing their magical connection to their body. This basically allows a Displacer to cut off their ear and give it to a friend in order for them to hear what their friend is saying to them while they're apart, or to survive having their "capa 'detated from their head". 
  The problems with putting this race in 5e should be obvious, the race would probably be immune to slashing damage, and the sheer amount of body parts that you could detach to serve different functions would make a "Displace Body Part" feature be very long, complicated, and likely overpowered.
• Dwarves turning to stone in sunlight. In Norse mythology, Dwarves turned to stone in sunlight (as did the Jotunn, which is why Tolkien had Trolls turn to stone in sunlight). The issues with this being added to 5e should be obvious. Dwarven PCs would hate being turned to sun during the daytime (including me, if I were to play a dwarf). Drow and Duergar PCs hate Sunlight Sensitivity, and that's just disadvantage on attack rolls and perception, not actually becoming petrified when exposed to the sun. 
• Non-Vancian Magic. Specifically, magic that gradually refills over time, like it does in many TV shows, movies, and book series. The issues with this are apparent, that it would make spellcasting have to be completely redone, spells would have to be substantially weaker and cost more, you'd have to be stricter about keeping track of time, and so on. There's just an evocative feel of a wizard casting a huge spell and then just having to sit back and do nothing for a bit until they regain enough magic to do simple, cantrip-level spells to aid in a fight. Unfortunately, that doesn't work in 5e or any other edition of D&D. Don't get me wrong, I also like Vancian magic, but I feel like an alternative could also be fun. 

Okay, those are my first few examples. Please give your thoughts below. I hope that this generates an interesting and unique discussion. 

Just off the top of my head, want Colossal and Colossal + sized monsters to return, and have some of the more powerful monsters scaled up. The Tarrasque, the highest level Krakens, and Tiamat should be Colossal size at least.

Love the Beyonders Trilogy. I'll just provide some from another of my favorite series, the Inheritance Cycle. While magic in Inheritance is similar to the UA Onomancy wizard, it has a more unique flavor that is kind of hard to capture in 5e. Also the fact that magic drains your physical energy and can knock you unconscious or kill you if you overapply. Also the fact that killing is easy. Obviously, the second one players would hate, and third one would just unbalance the game.

Quote from Third_Sundering >>

 The Amar Kabal are a race of "basically elves" that are great warriors with long hair that can live for practically forever, though they achieve this differently from Tolkienish elves. The Amar Kabal are at around the same rate as humans, but have fleshy "seeds" that grow from the back of their neck that will fall off of their body if they die, storing all of their memories inside of the seed. If the seed is later planted in hospitable soil, the Amar Kabal will regrow within a few months at the same age that they were when they first died (typically around 20).  The reason this race could not exist in 5e is mainly because of the issue of letting a player live forever as long as the seed isn't destroyed, as well as the thematic overlap with elves. Although I love this race in the Beyonders series and their lore, and I wish there was some way to translate them to D&D 5e, there is alas no way to do it without the race causing all sorts of problems. 

Honestly, this sounds more like a ton of campaign plot hooks waiting to happen. "As long as the seed isn't destroyed" isn't all that problematic, assuming this is a known fact in the world at large. Combat tactics and traps would be modified accordingly, magic and poisons would be developed with this in mind, the seeds could become a sought-out resource for a variety of purposes, for any adventure where time is a pressing issue dying will still effectively remove the character from the party, and so on.  

There's really a lot that can be integrated into D&D if you really want it to though. We've had an official D&D supplement for playing as dragons at one point. Third edition had rules for levels beyond 20. Non-Vancian magic is something I've seen people try to homebrew in every edition I ever played. There have been D20 rulesets created for Lord of the Rings and for Wheel of Time, to name just two non-D&D fantasy IPs. I think there are only three real limitations: creativity, the amount of work involved, and what you define as still being D&D (for some it's not D&D anymore if the magic is not Vancian, for example). 

Quote from Lyxen>>

Why is there a finger pointing at 5e in particular about this ? The thematic overlap with elves has never been a problem, elves can are are different in various settings, see Athas for example, or Eberron anyway. As for the resurrection capability, how is that a problem exactly in any edition of D&D ?

I'm not blaming 5e for anything. Why are you suddenly advocating for 5e when there was no one attacking it? I was merely giving an example of a fantasy race from one of my favorite series that I wish could exist in D&D, but sadly cannot due to balance reasons. It's just due to how D&D works differently from this series that causes this inability to exist in 5e. I'm not blaming WotC, D&D, or the mechanics of the game for this. 

Again, I fail to see what the problem is here, with 5e or any edition of D&D. You can create these as monsters any time you want, just put that power in their statblock.

OK, at this stage, if it's just a question of power, I agree that the mechanism of ECL that existed in 3(.5)e has not been replicated in 5e. But it's just because there is no need for it, as it is up to the DM to balance things any way. You can create monstrous adventurers, and you can compensate large powers with no ASI, no other special features, etc. And compensate for other characters if you allow one to play as a race that has specific advantages.

You're going to have to tell me what ECL stands for, as I'm not familiar with it. As I stated clearly in my post about the Displacers, an issue with putting them in 5e would be because of their power and because of how complex the rules would have to be for it. 

Again, what's the problem with 5e specifically and D&D in general ? It's unplayable in any campaign in which you have a chance to see the sun anyway.

The problem is that Dwarves work one way in 5e, and D&D campaigns often have players in the sunlight, and this idea that I think would be cool is incompatible with those two things. I am confused as to why you're arguing with me. I am not attacking D&D as a whole, WotC, or 5e, and even if I was I would not be attacking you, so there's really no point for you to be so outright hostile.  

As usual, it's not that it cannot be done, for example, just design a sorcerer with no spell slots and just sorcery points, and have sorcery points regenerate over time. The problem is not 5e in itself, if you are creative, it's balancing of powers in what a lot of people want to be a technical game. But if you are less concerned about technicalities, and more about stories, it will work fine.

I'm not saying that it can't be done, I'm saying that it's incompatible with the current rules and would be incredibly difficult to implement in a balanced way. 

Honestly, there are tons of powers from books and movies that would be terrible in most roleplaying games, especially technical ones, because the problematics are not the same at all.

Which is entirely the point of this thread. It is completely the point that there are a ton of powers, races, and other cool aspects in books, movies, and TV shows that sound like they would be cool to add to D&D, but cannot work for a variety of reasons. The point is not to debate them or attack other people's suggestions, the point is to give examples that seem cool but don't work in 5e (or D&D/TTRPGs in general). 

But balancing a mistborn character is impossible. Ot a Surge Binder. Why ? Because the limitations are not linked to fatigue, long rest, short rests or whatever, but to the availability of external resources, like spell components, metals for a mistborn or stormlight for a surge binder. And in stories where these are limited, where these resources can be targeted, drained, in dramatic moves, it works well. But unless you are playing a very free form game (and there are a few that work very well), you will not achieve the technical balance that you need for a balanced D&D game, independently of the edition.

Now this part of your post is on topic for the thread. Powers being based off of metals that are consumed or used to focus your powers is a really cool idea that can exist in a book series, but not in D&D. Stuff like this is welcome to the thread. The rest, however, is not. 

Now, about your "disclaimer", report me all you want, but honestly, why are you creating things like this with "impossible to do in 5e", instead of creating POSITIVE threads like "I have seen this cool creature in a book, how would you implement it in 5e, I can't find a way" ? Why rub people who actually like 5e the wrong way when there are solutions and especially when the problem is not linked AT ALL to 5e ?

Because sometimes it's okay, or even constructive, to talk about things you wish a hobby could have, but can't, even if that is in no way the fault of the hobby. Take prophecies/oracles as another example. I love prophecies in books. Not when it's literally spoiling the whole thing or railroading the book characters, but like in the Percy Jackson (and Heroes of Olympus and Trials of Apollo) series, where the prophecy is more vague and can have multiple outcomes or interpretations, instead of just being the plotline for the adventure. 

The problem with prophecies in D&D is, of course, that players get to make their own choices in the campaign, unless the DM is railroading them the whole time and they have no player agency outside of what they do in combat. Vaguer prophecies can work in D&D, but they're difficult (practically impossible) to make foolproof of player agency. 

Prophecies are cool, and I wish there was an easy way to incorporate them in D&D, but due to the nature of the game, it's typically better to leave out prophecies than include them. I'm not blaming D&D or the game designers for this, it's just an unintentional side effect of how the game works. That doesn't make it bad, it doesn't mean that you shouldn't play D&D, and saying this definitely is not me trying to "rub D&D players the wrong way", it's just me talking about how some aspects of fantasy stories are sadly incompatible with D&D. 

Letting my video game love show here, I’d love a more complex crafting system that incorporates different creatures, plants, and other materials to create interesting and unique items both mundane and magical with interesting properties based on the items used, like Monster Hunter or Elder Scrolls.

I’d also love to see monsters break the 30 CR cap, and have stats that exceed the 30 in any stat cap as well. Creatures that are epic tier that you have to go on epic quests to prepare to take one of these creatures down, or creatures that encourage different approaches to defeat or fend off due to being so powerful. 

Quote from RoughCoronet0 >>

Just off the top of my head, want Colossal and Colossal + sized monsters to return, and have some of the more powerful monsters scaled up. The Tarrasque, the highest level Krakens, and Tiamat should be Colossal size at least.

 Gargantuan monsters have no size cap, you can make them 100' x 100' if you want. My problem with giant monsters in 5e is mostly that they don't feel very impressive (This is a side effect of the general power level flattening in 5e; you can't make it so orcs are remotely relevant to level 20 PCs without also making low level mooks relevant to CR 20 monsters). 3.5e was a mess in a lot of ways, but its giant monsters did feel suitably excessive.

I think most things are possible in 5e, but many things are improbable due to complexity or being outside the bounds of the limits of the game.  One thing I'd love is a more developed familiar system that would allow for greater customization and options.  It would make it easier to incorporate concepts like Pokemon or the Blades in the Xenoblade series into the game.  The issue is not one of feasibility, but of complexity.

Pie-in-the-sky desire?  I'd love a point builder version of D&D, with no races/species, no backgrounds, and no classes.  Instead, everything gets a point cost and you get a budget, and the sky (and what your DM is prepared to accept in their world) is the limit.  Let creativity roam free, let people build some truly incredible stuff, and play the stories they want to play.

But...well.  Heh, odds of that happening?  Zero.

In less blow-up-D&D-as-we-know-it wants, I'd like for the system to handle unconventional body shapes better.  The DM for my Friday game created a naga/lamia-like species of Half-Snek Folks called archons that were supposed to range between fifteen to twenty-five feet in length with either a constriction attack or a toxic bite depending on subspecies, but D&D simply cannot handle any PC that isn't a standard bipedal humanoid build with two hands, two feets, and which can fit entirely within a five-foot cube.  One can temporarily assume such forms, but the game assumes those forms are exactly that - temporary - and thus does not allow PCs to accomplish much of anything except direct combat without being bog-standard ordinary Meat Dude.  A PC with more than two manipulators immediately explodes the playerbase into nuclear annihilation, and any PC with fewer than two manipulators tends to do the same as everyone shrieks about someone "GIMPING THE PARTY(!!!)"

I'd love for D&D to be able to say "No sweat" to Tiny pixie-like fey PCs, twenty-foot naga/lamia Snek People, Awakened hounds or other quadrupedal critters, or even just handling centaurs properly, rather than being super terrible at all of the above when it doesn't just say "NO" outright.  Similarly to the new dual-type Lineages, which the system handles very clumsily, I wish the game was better about working with nonstandard PCs.  Anything beyond "human, just a different color and with a few extra cosmetic bits" comes apart and it's super sadness.

Oh well.  Perhaps something for the future.

Shopping malls (I am doing the Internet and adventurers as celebrities in my FR campaign). Actually, now that I think of it I could add a mall to a cosmopolitan city setting like Waterdeep if I really wanted to...

Also I really wanna update Al-Qadim to 5e. I only ever read the Land of Fate PDF on DTRPG, but it seemed like a really cool setting and I loved the fact that they made the Loregiver (the prophet of the enlightened gods) a very wise teen girl.

Quote from Yurei1453 >>

Perhaps something for the future.

Eh. Just do it. If it works for you and it still feels like D&D to you, why let the idea that it might rustle some other people's jimmies stop you? Awakened animals rather than more anthropomorphs sounds great. If I bring up I don't have a problem with some players having significantly better stats than others or that I can even that out by giving the PCs less fortunate in the ability mod department powerful items there's people who insist I'm doing a Bad Thing without knowing anything about me, my players or our campaigns. They can believe whatever they want, it doesn't affect what happens at my table. D&D can work fine with nonstandard stuff. Scratch that - D&D can be awesome with nonstandard stuff. You want to be a giant snake with a PhD in philosophy, recently gone vegan and recurring weird dreams the gods becoming mortal? Go for it! Rather want to be the spirit of a deceased pharao denied entrance to the afterlife, bound to another PC who's been cursed with your haunting presence, and only able to affect the physical world 7 times per day? Why not, sounds like a blast. Floating three-headed frog from the Astral Plane with a fetish for male fire genasi and a financially crippling addiction to poetry tablets from some long lost civilization of seafaring nomad dwarves? Roll on!

Quote from pangurjan >>

Quote from Yurei1453 >>

Perhaps something for the future.

Eh. Just do it. If it works for you and it still feels like D&D to you, why let the idea that it might rustle some other people's jimmies stop you? Awakened animals rather than more anthropomorphs sounds great. If I bring up I don't have a problem with some players having significantly better stats than others or that I can even that out by giving the PCs less fortunate in the ability mod department powerful items there's people who insist I'm doing a Bad Thing without knowing anything about me, my players or our campaigns. They can believe whatever they want, it doesn't affect what happens at my table. D&D can work fine with nonstandard stuff. Scratch that - D&D can be awesome with nonstandard stuff. You want to be a giant snake with a PhD in philosophy, recently gone vegan and recurring weird dreams the gods becoming mortal? Go for it! Rather want to be the spirit of a deceased pharao denied entrance to the afterlife, bound to another PC who's been cursed with your haunting presence, and only able to affect the physical world 7 times per day? Why not, sounds like a blast. Floating three-headed frog from the Astral Plane with a fetish for male fire genasi and a financially crippling addiction to poetry tablets from some long lost civilization of seafaring nomad dwarves? Roll on!

 I like it!

Quote from PenelopeTheWeaver >>

Shopping malls (I am doing the Internet and adventurers as celebrities in my FR campaign). Actually, now that I think of it I could add a mall to a cosmopolitan city setting like Waterdeep if I really wanted to...

Yes. You absolutely could. I have a massive bazaar in the capital of the main desert nation in my setting.  

Quote from pangurjan >>

Quote from PenelopeTheWeaver >>

Shopping malls (I am doing the Internet and adventurers as celebrities in my FR campaign). Actually, now that I think of it I could add a mall to a cosmopolitan city setting like Waterdeep if I really wanted to...

Yes. You absolutely could. I have a massive bazaar in the capital of the main desert nation in my setting.  

 Cool. Thanks 😊.

Mechanically i would love some of the higher CR creatures. Things like Greatwyrm dragons, or the extraplanar beings that have actual stat blocks so that if someone wanted to make a godlike creature we have more then one to biased the rest of our post level 20 fights on. More things for that level of play in general would be great too. Cr is kind of a joke, because if you don't play the creature perfectly the party can completely destroy the creature easily, especially at level 20. So really just creatures beyond CR 20.

I would also love a way to easily bring stuff from older editions into the modern game. Be it mechanics or otherwise, having some sort of transition guide would be great, especially for the classes and creatures that I had been missing out on.

Quote from Yurei1453 >>

I'd love for D&D to be able to say "No sweat" to Tiny pixie-like fey PCs, twenty-foot naga/lamia Snek People, Awakened hounds or other quadrupedal critters, or even just handling centaurs properly, rather than being super terrible at all of the above when it doesn't just say "NO" outright.  Similarly to the new dual-type Lineages, which the system handles very clumsily, I wish the game was better about working with nonstandard PCs.  Anything beyond "human, just a different color and with a few extra cosmetic bits" comes apart and it's super sadness.

Oh well.  Perhaps something for the future.

 A lot of those are less "impossible" than "5e didn't bother to figure it out". You could perfectly well implement a 5e equivalent of Savage Species, it would just be a large amount of hard to balance work that most players would never use, so they didn't bother.

It can be done, yeah.  It's more a case of "too impractical" than truly impossible.  Maker's Archon species already exists in homebrew, we just handwaved the weird size nonsense that arises from having twenty feet of snek butt hanging off your ribcage that says you should be Huge or even Gargantuan despite being mostly just regular dood-sized.  It works, but we all know it's a jank workaround.  Similarly for a lot of the other nonstandard options - stats can be written without much issue, but a lot of the game's core assumptions go out the window and a DM will constantly be adjudicating tangential weirdness related to your off-base species option.  Which some tables are perfectly cool with, and awesome - more power to 'em.  I know my table would likely be fine with much of the stuff I listed above if I could work up the right story for it, but man.  Lots of barriers to entry, even for relatively proficient homebrew folks.

I would like to throw my vote in for PC's outside the small/medium range. We had them in other editions and we could in this edition if WotC actually tried to create the rule set instead of them doing everything in their power to avoid creating rules for the game.

I also want more freedom and customization. The class system feels very rigid to me and it is not something I am fond of. If there was no level limit and I can multiclass into the same class (to pick up other subclasses), that would be alleviate some of the rigidity issues.

If I want to make a wizard who is a master of all schools of magic, that requires going above level 20 and multiclass into the same class. While that is relatively easy to achieve using pencil and paper if you are used to manually creating your character sheets, it is harder to do that on Beyond with the homebrew tools if you are to using the character builder.

Psionics that anybody of any class can acquire. 

There would have to have some form of cost or sacrifice made so that it wouldn't unbalance classes, and making it a class of its own is NO.

While I also want to say fewer caster subclasses, it's probably just the old man in me crying about how it used to be.