My friends and I started playing DND this year. Most of us heard about it and have some general idea of what it's about.
So we spend probably a month? to reading and researching. And I was given the DM's Staff of Infinite Power, for me to create and abuse.
It seem to be an incredibly daunting task back then - but in hindsight, after 7 months of being a DM, does not seem that bad now.
There are so much rules, details and feature. I don't know where to start. 
So to save me some trouble (partly also being lazy and slightly ADD), I gonna create a simplified world. One world with limited races and little to no magic. And my first thought of a world setting is SAO. My friends likes it, so I continued from there.

I restricted the choice to just 4 races : Human, Elf, Dwarf, Half Orc, Half Elf.
And initially no magic class. But I expect to get better in DMing, so I opened up a little, there is some magic.
The process of simplifying the game somehow becomes one that I have a lot more work, on the homebrewing side (I come to that later.)
But overall, I have so much fun, designing the world, coming up with lore, incorporating the player's backstories into the world and lore. It's fun! Did I say fun already? Yes It's fun.

So the gist of my campaign goes like this:
6 years ago, a mysterious Tower appears. And since then the world is not the same. Strange disaster and phenomenon force everyone to seek refuge in the center, and ironically around the Tower. Monster grew larger and more aggressive, Weather are extreme in the fringes of the world, and magic - which used to be a novelty ability- becomes stronger.
You start off in the only city left, built around the Tower call Arinsgald. You signed up to be a party, under the care of the Adventurer Guild, where you can receive quests and rewards. What spur you to do so is because someone went into the Tower and defeated the first floor. Before that, no one had ever came out, they went in and disappeared.
Your quest begins as you seek to gain more experience and one day, enters the Tower and reach the top, to solve the mystery of the Tower.

I figure, with this world mechanic, using the Tower as a way to increase the world difficulty, and also add in more feature is one way to help the players get used to DND. But I now find it redundant. For 1: I severely underestimated our ability to figure things out. And 2. My players are extremely cautious, so progress is relatively slow. I have set 20 levels for the Tower's progression, but so far (with 2 groups running) we are only at lvl3...

As mentioned earlier, what initially is simplifying the game became more work. After I mentioned that they are some magic, players start to ask whether they can play Cleric, Paladin, Bard, Sorcerer, etc. And I have to thing about how the class can fit into the world. Which I allow for the mention also Druid, but nor Wizard and Warlock (for now). Hopefully none of my players in on the forum as this is some major spoilers, haha. Yes, for allowing them to play a Magic class in a a world with little to no magic, I told them that their magic will be nerfed. And that... did not stop them. So here I am, looking through ALL the spells and reworking everything. Certain spells are impossible, some spells are rework (either combining 2 or more spells to one, or changing the description). It's a lot of work, but it's working surprisingly well.
Using Bard as an example.

• For every spell cast, the Bard have to make an additional Charisma Check, the roll will determine if he succeed or fails; if he succeed, is it a poor, fair or good cast.
  a poor cast would have some form of detrimental effect (-1dice roll, disadvantage on concentration check, halved damage). If it's a good cast, opposite of poor cast, and the spells have a chance to "level up" or unlocking new spells. It's a Massive change to the 5e, but it made to fit into the narrative.
  
  
• I also allowed the Bard to cast 2 spells that require concentration, and even a third spell without concentration while maintaining 2 spells. The logic behind this is that the Bard is playing a melody/chord on his lyre to cast spells, and sometimes a magician can play multiple pieces simultaneously. It in a sense is a huge buff, but he have disadvantage on concentration check, he can't move, and so on.

But I am planning for an overhaul somewhere further down into the campaign. I already set up and design that encounter, and after that. All races and spells are accessible without restriction. And I can't wait for that to happen!

Another thing that I added into the game as I was doing my research, is an AC system I found online. Which introduced the dodge mechanics and Armor Protection.
Instead of a flat AC for hit/miss. There is Dodge(0dmg), Glancing Hit (reduced damaged) and Hit (full damage). It took awhile to balance everything. But it is working now.

Probably what I am doing as a DM is sacrilegious to many in the community, but the 2 groups that have been with me are enjoying the game. So I hope that justify it.
So what do you think?

First off, making an Aincrad-themed campaign is something i've been thinking about for a while. Second, if you want to make a world with low magic, I suggest instead of nerfing every spell (which is a complete pain) you could also just limit the casting classes to ones with less access to more game breaking spells. For example, Warlocks at 1-3 lvl only have 3 spell slots. I consider them more a melee build, especially since the Hexblade subclass is it's own class entirely according to me. The sorcerer has some potential for lore if you explain their Innate Spellcasting with something similar to Solo Leveling (i.e. certain people are "reborn" with superhuman-like magic abilities.) This would really save you some time and work without remaking the whole magic system of D&D.

I really like the armor system you're using, it's similar to the one that I use and adds more realism to the game. Another just handy house rule that my table uses is Healing Surge. You can expend up to half your hit dice as a bonus action. Usable only once until you take a long rest.

I can see that you have a good grasp on the game already, especially since you've never played and are already trying your hand at DMing. (Respect) If you have any other ideas that need input, just PM me. (This is pretty much my dream campaign)  

There is a weird perception that people think they're going to be judged on forums like these for what they do at their table. I'm not saying it never happens, but I've never seen it happen.  I think most of the arguments about rules are about changes or proposed changes to the written rules. If you want to change AC or add spell checks or otherwise add qualified success as a general rule, then you do you. It's not substantively different than adding in homebrew rules of any kind, such as Matt Coville rules for organisations and warefare. And, if we're being honest, we have an entire section for homebrew on this website.

I'm glad a lot of these experiments have  worked out for you. Sounds like a really neat campaign. It reminds me more of Maka Toushi SaGa/Final Fantasy Legend than of SAO, but it sounds like you're having fun and your players are invested in the game. So, job well done (so far)!

Quote from FadedElysium >>

First off, making an Aincrad-themed campaign is something i've been thinking about for a while. Second, if you want to make a world with low magic, I suggest instead of nerfing every spell (which is a complete pain) you could also just limit the casting classes to ones with less access to more game breaking spells. For example, Warlocks at 1-3 lvl only have 3 spell slots. I consider them more a melee build, especially since the Hexblade subclass is it's own class entirely according to me. The sorcerer has some potential for lore if you explain their Innate Spellcasting with something similar to Solo Leveling (i.e. certain people are "reborn" with superhuman-like magic abilities.) This would really save you some time and work without remaking the whole magic system of D&D.

I really like the armor system you're using, it's similar to the one that I use and adds more realism to the game. Another just handy house rule that my table uses is Healing Surge. You can expend up to half your hit dice as a bonus action. Usable only once until you take a long rest.

I can see that you have a good grasp on the game already, especially since you've never played and are already trying your hand at DMing. (Respect) If you have any other ideas that need input, just PM me. (This is pretty much my dream campaign)  

 Thanks!
Initially my plan was to just straight out limit to just Paladin Class. But I find having a give-and-take attitude or relationship with my players is the best.
After all they are players in the world that I created. I certainly don't want to be a "no, no , no" kind of guy, so if the player have some ideas, I tries to see if it can fit into the game.

Since, it was already my plan to overhaul the world to have access to magic. I figure having this Tower to be a catalyst to magic increasing in power.
Initially, players using magic have limited spells and chances of failure, but as the story progress, they in a sense become stronger and more adept. 
I think the biggest change I did for the spell system is removing the needs for materials, and just be Somatic and Verbal, and I impose a requirement for them to Roleplay when they cast the spell (I don't enforce, some of my players are shy, but I love it when they at least try, and complain about being cringe, but they still like the experience); the dwarf Paladin begging to his God, Dragoth to give him strength; the Bard who came out a list of encouragement when casting Enhance Ability; the Sorcerer who practice the hand gesture from Doctor Strange and Naruto.

I love your idea for Healing Surge! Thanks! I might include in my campaign as a form of cheat death. 

Quote from Verenti >>

There is a weird perception that people think they're going to be judged on forums like these for what they do at their table. I'm not saying it never happens, but I've never seen it happen.  I think most of the arguments about rules are about changes or proposed changes to the written rules. If you want to change AC or add spell checks or otherwise add qualified success as a general rule, then you do you. It's not substantively different than adding in homebrew rules of any kind, such as Matt Coville rules for organisations and warefare. And, if we're being honest, we have an entire section for homebrew on this website.

I'm glad a lot of these experiments have  worked out for you. Sounds like a really neat campaign. It reminds me more of Maka Toushi SaGa/Final Fantasy Legend than of SAO, but it sounds like you're having fun and your players are invested in the game. So, job well done (so far)!

 

 Thanks! That's comforting to hear!

Allowing extra Concentration is very powerful, so beware.