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?