More thoughts on my home setting:
As cool as Planescape is, I hate the cosmology that goes with it. The idea that everything fits neatly into a plane of a specific alignment, that it’s set up as a giant wheel, that there are elemental planes, para-elemental planes, quasi-elemental planes, energy planes, etc. is both stupidly overly complicated, and for the most part incredibly boring!
Look at that mess! And somehow it all wraps together in Sigil...
Now, 4e cleaned things up quite a bit. The Mortal world has a pair of reflections, the Feywild and the Shadowfell, and they are pretty much what they sound like. Ravenloft? In the Shadowfell. The court of Oberon and Titania? Feywild. It fits together with the implied cosmology of most fairy tales so much better than the Great Wheel ever did.
The Astral Sea will be much like it’s usually portrayed, including dead gods, strange worlds, demi-planes, the Restaurant at the End of the Universe/Callahan's Bar/The Captain’s Table/etc., and of course Sigil itself. The currencies of the realm are magical power and souls.
What about the gods? I’m going to stick with the idea that there are no gods, per se, but rather the immortals from classic D&D. Mortals who have become so powerful they ascend to demi- and lesser-god like status. They live in realms that float in the astral sea, and interfere as much or as little as they like in the mortal realms. Clerics and other “divine” characters may worship these immortals, and their worship does have an impact on the immortals’ power and standing, but divine power does not derive from individual immortals, but rather from all of them. That isn’t to say that an immortal can’t focus on one individual and boost or block them, but it takes effort and energy on their part. This will allow divine characters to choose just about any god or ideal to worship.
Demons and devils? I’ll stick with the 4e idea that they all come from the elemental chaos. The upper levels of it resemble the classic idea of the underworld, and the deeper you go the more twisted and chaotic it becomes. Players will never visit the lowest levels, as reality breaks down if you go too deep, and this blends right into the far realm - aka outside the player’s universe. I’ll never send my players there, but it’s a great place for *things that are not meant to be* to come from.