One World Cosmology

[Trask]

On p.44 of the D&D 5e DMG in the section about the multiverse it outlines some of the different planar models you can use in your game. One that particularly stands out to me is the "One World" cosmology, where all the things attributed to separate planes is located somewhere on the physical world, like mount Celestia being an actual mountain or the Abyss lying deep under the earth. You might also call this model "mythic geography" since that line of thinking is common in old myths and legends.

I'm very attracted to this kind of world because of it's imminence and reliability. Walking through a portal into the plane of fire feels routine, but scaling a volcano and plunging into its molten depths to reach the realm of fire is awesome and inspired because it doesnt separate the act of traversing the supernatural from actual traversing, like walking, climbing, diving, flying etc. You can literally dig your way into hell or fly up to the realm of upper air and converse with the spirits of wind and that just screams to me that its perfect for actual "at the table" play and helping players find the magic in the world.

The question is, how best to implement such a planar system? The DMG itself outlines a checklist of fundamental conceits built into the bones of any D&D 5e world, and they are

- A plane of origin for fiends
Easy, it can just be The Pit, or some dying demon star, or whatever else.

- A plane of origin for celestials
Similarly easy, you could go for classic mount olympus, the stars, the sun, the moon, a mix of all, or anything else you want

- A plane of origin for elementals
A little tougher because we are avoiding separate worlds as much as we can, but you could easily say that elementals just exist as part of the elements they embody and "elementals" are just pulled forth from the raw matter, or maybe its that matter combined with a soul or spirit to give it animate form, but there are a lot of ways you can do it.

- A place for deities (which can include the first three)
This is infinitely flexible as are the concept of deities themselves. They could exist on the world, but remotely, or they could dwell in the celestial or fiendish realms respectively, although I think it might be a fun twist to keep the deities conceptually separated from LAW and CHAOS for once and have them act more as spirits.

- An afterlife (which can include the first three)
Also easy, although this is where traditional concepts begin to overlap with each other a bit to make things kind of messy. The classic concept of Hell is underground, but the same is true for Hel, or Hades which has a very different feel to it than a home for fiends, its really more like the Shadowfell. But you can still make it work if you tickle it a bit, although this is where I definitely see the advantage to the traditional planar model as it makes a lot of nice space to put whatever you want, the problem is they never stopped. But I think that the domain of fiends and celestials could possibly be the only exception to the rule here.

- A way to travel between planes
This isnt really necessary at all in this model, but I can see that this model does make some spells pointless (like Planar Shift) but honestly I'm ok with that, its easy enough to tweak in the game and I dont think it really changes much to not have those spells.

- A way for spells and monsters that use the Astral and Ethereal planes to work
This one actually does matter a lot more than the previous, but thankfully the Ethereal Plane is easy enough to explain, its simply the archetypal Spirit World, and maybe it can borrow some concepts from the idea of Faerie. Astral travel is more sticky, but I think I could explain it like traveling through literal space, or the outer air around the earth.

So taking all those assumptions what kind of world can we build here? Comment your ideas for a One World cosmology, also comment if you've ever used this model and what you thought about it.