D&D General Yggdrasil World Ash Tree and the Great Wheel Cosmology

[TheFantasyHistorianYT]

If you want the most complete and precise way of doing it, you'll want to use the 2e Planescape sources. 2e cosmology (ie, Planescape) basically assumes anything from a 1e source is true except where 2e sources contradict it. There are a lot of 2e cosmology sources so that ends up contradicting a lot of it. To cover all the Outer Planes well, you'll need not only the original 2e Planescape campaign setting, but also 3 other boxed sets (Planes of Law, Planes of Chaos, and Planes if Conflict), and the Planewalker's Handbook wouldn't hurt.

Since you will most likely be acquiring these from DrivethruRPG / DMsGuild (there is usually a big D&D sale at least once a year) you can search the pdfs for "Yggdrasil" to find all the info on where it connects. It doesn't attach to every plane in 2e, but it has connections to quite a lot of places.

3e took the 2e Planescape cosmology (itself a revision / extension of 1e's), called it the Great Wheel cosmology, and selectively changed some significant elements (like the Astral Plane and the quasi and para elemental planes).

4e did something completely different, and there are things you can take from it or import into it, but it's trickier than with other editions.

5e's cosmology is IMO, closest to 2e's, but stripped down, importing some content from every other edition (including 4e), and then adding its own innovations.

Unless you plan to run a game solely using a single edition's lore, you will have to make some judgment calls to resolve contradictions. My recommendation is to choose the lore of whichever edition you prefer as your baseline (this need not be the same as the rules edition you play, so if your players are familiar with 5e rules, but not completely drenched in its cosmology, it should be easy to choose any other edition's cosmology as your baseline) and then selectively import everything you like from other editions, which will mean replacing some of that baseline.

Since I feel like 2e Planescape is the richest and most comprehensive body of planar lore, I use it as the baseline for my 2014 5e D&D. I use most the additional material from 5e that adds to it, but rarely import things that contradict it, although the way I handle most 5e planar lore I don't like is with a "some people believe that" unreliable narrative angle. So most Prime Material Plane planar sages believe the multiverse looks like it does in the picture in the 5e PHB, while sufficiently educated but not expert Primes think it looks more like a simplified 1e Manual if the Planes, and only the inhabitants of the other Planes are likely to know that it really looks more like 2e Planescape.

And of course if you want to include Spelljammer in all of this (since it is integrated into the cosmologies of its editions) you have your work cut out for you, since it is very different in 5e than in 2e. I took the opportunity to to sort of merge the diverging views together into something more expensive, and ended up fixing some of the various elements if Spelljammer that always bugged me (though I like it in general).

I enjoy this sort of thing, and feel free to ask any more questions you might have about it.

That was awesome thank you! When I did an overview of the other settings, Spelljammer and Planescape seemed cool, but it wasn't until I looked at the World Cosmology that it clicked for me and I had that "Aha!" moment of how incredibly useful those other settings are and another reason of why they exist on top of just printing more settings. I am glad to see that I wasn't wrong in my "Aha!" moment lol.

I appreciate the advice on the judgement calls along with Whizbang Dustyboots advice on it. I am currently using 5e as a baseline just because it is the most familiar. Since Planescape is incredibly rich with lore, I have been using it with a bit of Spelljammer and the other Manual of the Planes to greatly enhance it. If anything contradicts with 5e, I take 5e over it, BUT I will add it if it can still fit somehow. Probably the harder way of doing it, compared to taking Planescape as the baseline as you mentioned haha.
-For example, I was thinking of adding the Quasi-elemental planes and since there were two planes with the name of "Plane of Ash", I ended up keeping the 5e Para-elemental "Plane of Ash" and swapped the Quasi-elemental one with the old Para-elemental name "Plane of Smoke."

Other things I have run into:
1. Different deities and the realms they control which may or may not exist in 5e. If I can add them without contradiction I will, but I am not focusing heavily on them anyway. Only if it helps giving an overall picture of a plane. Norse Pantheon on Ysgard for example I liked a lot.
2. 4e's overhaul, especially with the Abyss and the hiatus of the Blood War. I've pretty much just disregarded 4e except for other bits of useful information, especially that of Shadowfell, Feywild and the retcon of Ravenloft into Shadowfell. I plane on keeping the Fugue Plane though.
3. 5e Spelljammer threw me for a loop on the Astral Plane because I kept finding answers that said the Astral Sea is another name for the Astral Plane, but found out that the 5e Spelljammer Sourcebook says the Astral Plane consists of BOTH the Astral Sea and Wildspace. Phlogiston and the Flow are gone. Crystal spheres around the planetary systems have disappeared and Wildspace expanded. Now players can Spelljam their way straight to the Outer Planes without needing to find the corresponding colored portal in the Astral Plane?

Is there anything else I should look out for as far as contradictions that I can reconcile?

 

[Sword of Spirit]

Is there anything else I should look out for as far as contradictions that I can reconcile?

Seems to me like you're aware of the major ones (which means there are probably actually at least 2 other very big categories of contradictions I'm forgetting about at the moment, lol).

One thing you will also run into is that demon princess and fey beings and such were all given deity ranks in 2e, rather than arch-fey and arch--fiend being different types of godlike beings like in 5e. You'll have to decide what to do about that. I eventually ended making them all different categories of what I call Mythic Beings with different strengths and weaknesses. So deities are the most powerful in certain ways (that tend to be highly relevant to mortals) but not necessarily in every way. (Primordials and some Titans could take on Greater Powers in a straight up fight.)

One extremely useful book for the connections between the pantheons and the planes is On Hallowed Ground. One of my favorites. It actually tells you which deities from pantheon A are friends with deities from pantheon B, etc. It included the traditional D&D fantasy-historical pantheons as well as the 2e "single-sphere" (as in Crystal Sphere, so the specific campaign settings) pantheons and the non-human racial ones. And if you are liking that and want more info on the deities and pantheons, you'll want 2e Legends & Lore and Monster Mythology, because those are major sources that On Hallowed Ground draws from and extends.

Since you were noticing the Spelljammer redo, the real quick description of the much more detailed way I resolve it is:
1) Crystal spheres and the Phlogiston exist, and it is the most popular way to travel between the worlds.
2) The Phlogiston is a third region of the Ethereal Plane, that can be reached via the Deep Ethereal, but you can't really take a Spelljamming ship into the Deep Ethereal.
3) Some stars on the insides of crystal spheres are actually portals to the Astral Plane as in 5E Spelljammer.
4) The Astral Plane is less commonly used for transport between crystal spheres because it is riskier, but it is faster so it gets used by the brave or powerful sometimes. It's really hard/slow to take a Spelljammer ship to a color pool (and the ship won't fit through) so Spelljammer ships mostly just go from one sphere to another. You could hitch a ride to the Astral and then jump overboard and get yourself to a color pool though. (I also speed up most Spelljammer travel times, because they are too slow in 2e).
5) Instead of Wildspace being both the Material Plane and the Astral Plane as 5e says, it is the Border Astral Plane. Cosmologically it exists as part of the Astral Plane, so it doesn't directly connect to the Ethereal Plane for example, but its properties are more like the Material Plane.
6) Generally the planets that make up standard campaign settings are part of the Material Plane, while all the moons and asteroids and the occasional weird Spelljamming aware world are part of the Border Astral.
7) There are way more people living in regular Material Plane worlds than living out on all the weird Spelljamming places, but it doesn't always look that way because the weird sphere ones tend to get in the big Phlogiston rivers, while the more typical ones with large Material Plane majorities are off on the smaller flows.
8) This one isn't just my house rule, but is basically stated (though usually just in an off-hand way like everyone should just know it) in Planescape and Spelljammer lore: There are a lot of Material Plane worlds out there that worship the fantasy-historical pantheons, which is why they are considered multi-sphere pantheons rather than single-sphere pantheons like the Forgotten Realms. (I took this info and used it to inform #7 above). In fact, you learn from On Hallowed Ground that the most powerful and widespread pantheons are the Greek, Norse, Celtic, Elven, and Dwarven (the "Five Great Pantheons"). They have a huge number of worshippers across numerous worlds, as do the others like Egyptian and Finnish.
9) Finally, most pantheons don't like space invaders messing with their worlds (part of the reason they are off the major Flows), and space-farers know this, so they steer clear of Material Plane worlds and keep a low profile if they have to land on one. This keeps groundlings ignorant of Arcane Space in general, and explains why your standard campaign settings aren't at risk from the dangers of space.