Yippeeekayaaay muthafukaaa! I just had another epic brainwave. I've been thinking days about how to reconcile the stupidity of D&D's planes with what I was trying to do with my game. Then you reminded me of the image from Crono Trigger of the End of Time. There's a lamppost surrounded by undefined darkness. So I would like to propose the Lamppost cosmology.
[Before I get any farther, I'd just like to know that I'm tired and my brain is addled. So I will probably leave out many transitions. Hopefully you'll be able to see where I'm going.]
In case you're unclear about what I'm alluding to, I'm proposing the Lamppost cosmology as opposed to, say, the Great Wheel, World Tree, or World Axis cosmology. Because it's hard to understand how planes interact, these "cosmologies" are simply visual representations of relationships between planes. The thing mentioned earlier that I was trying to reconcile was the fact that alternate worlds connect in D&D via the Plane of Shadow. At first, it didn't seem there was any good reason why the Plane of Shadow, of all planes, should be the plane that connects alternate worlds. But then I realized, I was wrong to think of the Plane of Shadow as "something;" I should think of it as "nothing." The Plane of Shadow, then, connects alternate worlds in a similar way that outer space separates Earth from other planets. Unlike outer space, though, the plane of shadow sort of "borders" on reality, receives some of the traits of the realities that border on it, and therefore is not entirely nothing. Anyways, in this cosmology then the Plane of Radiance is the lamp, the outer darkness is the Plane of Shadow, and the lit area surrounding the lamp is composed of the other planes, like the Abyss and the Elemental Plane of Air.
Every separate world is it's own lamppost. Greyhawk is its own lamppost, the Forgotten Realms is its own lamppost, worlds created by the players are their own lampposts in the darkness. Technically, there is no effect that allows a character to travel from one lamppost to another. However, they might theoretically do so with a wish or miracle. However, every DM or player is the god of their own universe. Or truth be told, any DM or player can be the god of multiple universes or "sister" universes in a "family" of universes. In our game, for instance, the DM is god of the Forgotten Realms, Greyhawk, and all other official settings. Anyways, if a player and a DM agree to allow a character to travel from a player's lamppost to a DM's lamppost (or to force the character), then the travel can (or will) happen. A character that makes this journey gains the Scar of Destiny, which is a mark that he is sort of "maintained" in this world by the active will of the player and the permissive will of the DM. Hence why, if the player goes on vacation, active will is absent, and the character disappears (presumably back to their world of origin). Also, if a DM stopped permitting the presence of a character, he can force him out (which allows a DM to scar port a player who has dropped from the RADAR). This cosmology is even more like Crono Trigger in that the iron gates imagery would also be present, as characters can only enter a lamppost cosmology at the discretion of the DM (who we could call a gatekeeper, among other things).
[It's worth noting that I don't think the divination features of the Scar of Destiny have worked in the previous game, so those will probably be dropped, but we can discuss that elsewhere.]
Anyways, that's the Lamppost cosmology so far. With it, I've explained the Plane of Shadow. There are two other planes though that are important enough that they also need explanations. The ethereal plane I think should basically be the Spiritual Plane. It's where ghosts lives and it overlaps the material plane. We might consider having the Weave actually be material on this plane.
This astral plane is somewhat more interesting. I think it should be THE mental plane. In other words, the astral plane is my imagination, your imagination, and the imagination of all the players. In many ways, the ethereal plane is to the PCs and NPCs what the astral plane is to the players and DM. The astral plane is where the minds of the DM and players live. In colosseum worlds, the DM or a player acting as a DM usually make their minds visible to characters in the form of avatars (which are invincible and omnipotent). The astral plane, which is where ectoplasm comes from, might also be the kind of substance from which the DM forms the other planes.
Player avatars or minds do not need to travel on the Plane of Shadow to "go" from their world to play in our world, even though the characters they control do. Players interact with our world not through the Plane of Shadow, but through the astral plane.
In this schema, I think colosseum worlds are in fact full lamppost worlds, but they are usually very small (unlike some material planes, which have infinite boundaries).
Anyways, that's all I've got for now, lol.