Though I initially wrote it for 4th Edition, Josh and I have been going over the notes and trying to create a largely system-neutral world bible, as I want it to be usable by people using 4th Edition, Dungeon World, D&D Next, and probably other fantasy role-playing games that we do not play.
During this process we have been revamping the world, such as it is, quite a bit. At least part of the premise--that the gods and primordials fought, but no one won--is still there. The other part, which could be summed up as, "Dungeons & Dragons, but in the Astral Sea", is where we diverged: we felt that sticking everything in the Astral Sea, though visually awesome, was limiting, and made it difficult to rationalize where people obtained things like food and materials.
Also you needed a very expensive, magical ship to get around.
The world now is the result of every plane collapsing into each other after the Sundering. A lot of it is covered in ocean (mostly because of the Elemental Chaos/Inner Planes grossly upsetting the balance of everything), but now it is easier to explain how people grow food, get water, and find materials to make things, though there are still crazier elements, such as the shattered trunk of the World Tree in the center of existence, flying islands and celestial dominions, god-corpses, and so on.
Elements from other planes get mixed in, too, so where the dark influence of the Shadowfell settled, it created Shadowlands that draw in undead and restless spirits. Undead kings, which could be death knights or liches, rule kingdoms of undead vassals, occasionally sending forth black ships to claim lingering souls and bodies to expand their ranks. On the other hand, the verdant light of the Feywild causes plants to flourish where none should, animals and plants to possess uncanny intelligence (and sometimes speech), and in some cases time to flow differently.
Some choose to remain on the world, protecting worshippers both alive and dead from the many threats that abound, not least of which are devils, because the Nine Hells is a place on a Sundered World: a massive iron sphere, it floats above twisted spires of iron that jut from the ocean like black, jagged teeth. A lattice of iron bridges connect the sphere to the towers, which teem with slaves, infernal citizens, and souls of the damned (or unlucky enough to get picked up by fiendish or undead raiding parties before the angels found them, though how unlucky that is depends on the angels).
And at the furthest edges of reality is the Far Realm. Creatures that venture too close often attract the attention of eldritch horrors, though even those that escape are often driven insane or mutated.
So, that is one part of the bigger picture. There really are not other planes, which might be a feature or a bug depending on how you look at it (I remember there being a huge uproar at 4th Edition's cosmology). I think that by moving the setting from the Astral Sea to something a bit more normalish makes it a lot more accessible; when I first ran A Sundered World I just gave Josh's character an astral vessel, because if I did not then it would have made things a lot harder for them to get around and adventure.
The only part that I am a bit fuzzy on is how the days and nights work. Originally there was no day or night because the Astral Sea just kind of had a default luminescence, but I am thinking that maybe it could lighten or darken regularly, giving some semblance of night and day. Some regions could also just be lighter or darker (having bits of Feywild or Shadowfell layered on, but not too much), or even go with something a bit crazier like having angels that formerly served the sun god keep towing it across the sky.
What do you think?