So my idea for what the planes should be like is very similar to the Great Wheel cosmology. However there are several changes as well.
- Limbo connects to all of the chaotic planes. Except, perhaps, the planes that are only secondarily chaotic like Carceri. Furthermore, Limbo has it's own layers, including the realms of various chaotic divinities. These are not the roiling maelstrom that Limbo proper is, but are all still quite chaotic in they're own ways.
- Olympus is chaotic neutral (good), like Ysgard. Olympus may be on the same layer, or a seperate one, I haven't yet decided. Arborea remains Chaotic Good, but may be undergoing some changes, like the whole plane may be getting the infinite layers deal that the Abyss has, with Arborea just being one of the layers. It may or may not be the uppermost one.
- The Beastlands are very similar, but the various Animal Lords & intelligent animals may be moved to the Outlands, or perhaps scattered throughout the planes, with the plane (or at least some layers of it) retaining the general wild feel. That region of the Outlands may get the name Beastlands, with the plane of Neutral (Chaotic) Good getting a new moniker.
- Not many changes to the lawful planes so far. One thing is that I may deemphasize Acheron's focus on battle. Also Grummsh and Magulybiet may be moving to the Abyss and (back to) the Nine Hells respectively. They're armies still battle, but largely as part of the larger Blood War. A significant fact is that since orcs and goblins are the same thing in my campaigns, they compete for worshipers. Thus they're relationship is altered somewhat.
- Outlands may have other layers. For instance Illsensine's realm may be on a separate layer (if I decide to keep Illsensine at all. If I do he may also be on a different plane and may be a different manner of being, like a "Great Old One" rather than a god), as may be Tír na nÓg.
- In addition to relatively normal worlds like Oerth and Abeir-Toril, material planes (not just demiplanes) may have wildly different rules and attributes. Oerth and Toril exist in a universe much like ours (I'm not using the crystal spheres idea from Spelljammer), but with more magic. In fact, like in the Spelljammer setting they may share the same universe, but others like Athas and Krynn would not. The Elemental Planes may just be a series of alternate Material Planes dominated by a single element, but I'm leaning towards having both the "main" Elemental Planes and element-dominated Material Planes. I'm changing around the Ethereal Plane. The Ether is part of all worlds, but the Ethereal Plane is not it's self coexistent with those worlds, instead merely connecting worlds.
- The Astral is in fact coexistent with the Material, but not the Outer Planes. So if you Astrally Project you start off in the part that is coexistent with the world and can from there travel to the higher spiritual planes (i.e. the Outer Planes). This is also the realm of ghosts and other spirits, the "Spirit World" of Oriental settings. The demiplane of Dreams, if I decide to use it would also be in the Astral Plane. So it basically takes the role of the Ethereal in many ways, as I judge to make more sense.
In general I'm looking to shake up the planes, so that there is more room for new material (though there was already a lot of room). My policy is not even the most knowledgeable of sages in places like the city of Sigil know everything about the planes (as much as Sigilians like to deride Primes as "clueless"). The Great Wheel is just one of many models for mapping out the planes, no more valid than any other. They may know that there are two layers in Bytopia, or three in Hades but there may be others that are uncharted.