First off, I would like to say how much I respect how 5E has been doing to make things a bit more expansive and touching on quite a few cultural inspirations. With that out of the way, I noticed some things that haven't been touched yet. And by this I mean it hasn't been made officially by the D&D dev teams, it might show up in homebrew but this is about the Official and Unofficial stuff put out by the D&D development teams.

First off, I noticed that Geomancer is not a Class or Subclass for any of the casting classes, I haven't heard about it being in any UA either, or seen it in any UA I've managed to get a look at. To be honest, I think the Geomancer archetype is mostly overlooked for various reasons, I think it's mostly because people think that Clerics, Druids, Sorcerers, and Wizards can do the whole geomancer thing with little effort but that's shortsighted and only has to do with surface-level stuff regarding the Geomancer. When someone says Geomancer, most people will immediately jump to the idea of a spell caster that manipulates earth and stone, but that's only the most noticeable parts of Geomancers, and often the only thing anyone sees in media depictions when a Geomancer does show up. Geomancers are far more nuanced than just some magic user that can manipulate the terrain. Back in college, I read a book that talked about the practice of Geomancy in China (I can't recall the book at the moment, it will take me some time to unearth and provide the title and author), and in that book, it talked about how Geomancers communed with the spirits of the deceased to find the place to bury the body because the spirits of the dead can be very particular about where their mortal body is buried. In a sense, Geomancers are kind of somewhere between a Druid and Wizard (with some hints of Cleric sprinkled in) and this archetype just doesn't get treated right when it does get attention in the modern era. I suppose Geomancers in a Fantasy setting get a similar treatment to Necromancers, they both undeserved reputations that stereotype them into a very small box that's only a fraction of what their archetype is truly capable of.

On the subject of Necromancers, they are an archetype that gets improperly typecast just because their magic specializes in interacting with the dead, the most overt part that people immediately jump to is their ability to raise and control undead, Necromancy covers so much more than that, and the mistreatment and/or desecration of the dead is more a cultural nuance than a definitive rule most people seem to treat it as. Different cultures have different ideas of what constitutes honoring and/or desecrating the dead. Maybe a Necromancer is doing what they do because it's part of honoring the dead in their society. Necromancy has been typecast as the "evil" magic in Fantasy settings for decades (if not more) but when you look at the rules for Necromancy and the spells it has, especially in games like D&D, you can see that distinction is more of an artifact of past eras than anything genuinely definitive.

Now, let's move on to monster archetypes. In particular, trolls within the Fantasy setting, especially games like D&D. Trolls in D&D are towering brutes that lumber around that aren't very smart and are more inclined to try bashing to two rocks together because they like the sound or eat anything that was moving but stopped for whatever reason than do something unexpected. That archetype is fine, for the most part, but it's far too narrow. In Scandinavian lore, trolls have much more variety, and I find it thoroughly disappointing games like D&D haven't given trolls more variety than that in the entire time they have been around. In Scandinavian lore, those sorts of trolls do exist, so we can keep them in our games, but there are also trolls that are a bit more tricky and cunning, some trolls even have the ability to mostly mask their appearance to mortals (they can't hide their tails when transforming because that's the primary focus of their magic and its loss in any way could disrupt their magic). Swedish lore even talks about "trollbloods," people born with troll ancestry that are naturally more powerful in the mystic arts than a typical human, which sounds like a great Sorcerer's origin to me, but the kinds of trolls that can do magic and hide their form would have to be established in the game, and possibly its lore, before that type of sorcerer makes an appearance.

Another monster archetype I have yet to see really touched is Demonic Dragons. Hear me out, Demons and Devils are usually pretty dangerous, and dragons are often referenced as the most powerful beings. But there's stories out there, especially world lore and mythology, that have dragons that are also classified as demons. The Demonic/Fiendish Dragons could be a whole new category of dangerous monsters for players to deal with. The most notable Demonic Dragon I can think of off the top of my head is the Vritra from Vedic and Hindu mythology, it's literally described as being a demonic dragon with a snake-like body. And this thing is no joke, it's literally described as being so powerful that an entire party of gods couldn't kill, just put it into a heavy sleep and seal it away with safeguards to keep it in that slumbering state. So, basically, the Vritra makes the Tarasque look like a mid-boss, I would say more but I've ranted about that in some other posts already and it would be more impactful if you researched it yourself.

So yeah, there's a lot of untouched and untapped stuff out there that really deserves more attention.

I enjoyed reading this, thank you.

I've been wanting to play a necromancer for ages. I have this concept of a village or town of gnomes who honour their dead by cleaning their bones, inscribing them with runes, having a party to celebrate their lives... then raising them to perform dangerous/unhealthy tasks such as defence/mining.

Geomancy doesn't really fit with D&D's standard magic system.  It would be mostly ritual magic, from what I understand of it.

There was a Geomancer prestige class in Third Edition, but it was actually for druids and revolved mostly around gaining an increasing number of weird and freaky mutations as you leveled up by growing parts from different animals and plants, like your hair turning into leaves or you gaining a wolf's bite, a giant ant's stinger, up to potentially getting a unicorn's horn so that by the time you reached the end of the class you looked like a rejected Teenage Mutant Ninja Turtle character.  And could potentially make something like eight melee attacks per round.

Anything an Oriental Geomancer could do, a Druid could do, and more besides.  Necromancers already exist, as Wizards who study the school of Necromancy.  Nothing needs to be changed.

There are hundreds of monsters to choose from.  Trolls are mostly known for their regeneration, and that is something no player character should have.  There is no need for trolls with any more special powers.  If you really must have one, you can justify it as something a Hag created.  Sorcerers can have almost any creature be the source of the blood that gives them their magic.  No need to change anything.

We already have Demonic Dragons.  Or at least Diabolic ones.  Timat, the queen of evil dragons lairs in the Hells.  She's a deity, or close enough to one that it makes no difference.  She's firmly in the realm of Things The Players Can't Fight.  If you want a demonic dragon from the Abyss, go right ahead.  Anything you can imagine can come from the Abyss.  If you want the critter that gnaws the roots of the World Tree to be there, do so.  You want something bigger and more dangerous?  Have the players run across a dragon so large it snacks on planets and has entire cities carved into its teeth and filled with demons of all sorts.

Elemental stuff in general is overlooked and ignored in DnD.

Three of the four elemental sorcerers got canned, despite it being a textbook class to get its power through elemental bloodlines. Cleric managed to get air and fire themed domains, but nothing earth or ocean based. I'm still waiting on an elemental paladin or ranger, as a half caster elemental gish is my ideal character which I can't even make effectively. Genasi are considered by many to be the worst races in the entire game from a mechanical standpoint.

So the idea of ever seeing geomancer as a full class, or even as a subclass of anything seems like a longshot sadly. Hell, even core rulebook classes from previous editions haven't managed to get in as classes this edition.