Judging from the article I've read at WOTC's D&D site, it looks like they are planning on returning to using Planescape-style old-school celestials and fiends! And also at least some of the Great Wheel planes are being used as well. Anybody got more information than I have? Here's hoping that they are going to restore the entire Great Wheel cosmology and creatures (and, dare I pray, Sigil?)
5th Edition Planescape elements
My understanding is that the Great Wheel is the go to example...it's a little different than a default cosmology as I think they'll be trying to draw from the Axis as well.
But yeah, there should be new stuff for the Wheel and IIRC the Blood War.
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It generally mentions the Great Wheel with the additions of the Far Realm, Feywild and Shadowfell thrown in.
Succubi are probably going to be Demons again, though the survey shows people are equally divided in whether they should be Demons (slight majority), Devils or something else.
They might bring back the celestials, but 4e angels and elemental archons might still remain too. The survey on that subject might have it's results today or tomorrow, which might be something to see, if these surveys have any influence on the story elements.
In addition The Ecology of the Modrons has a new plane. There's so many different cosmologies right now they might keep everything mysterious. Sigil was in the 4th edition, one difference I remember it had a new guild called the Arcane Brotherhood.
By the Nine Hells, I didn't mean to double-post!