I mentioned earlier that I think that this will mainly focus on the Outer Planes, and leave the Inner and other planes basically to be fleshed out elsewhere. Honestly, with just 96 pages (barring anything that could appear in the adventure book), there really isn't going to be room for much outside Sigil and the Outer Planes. My guess right now would be roughly 1/8 general setting info, 5/8 Outer Planes, and 1/4 Sigil. 24 pages for Sigil is roughly about what Waterdeep and Sharn got in their books, after all (and I would guess that some Sigil locations will appear and be described in the adventure). 60 pages for the Outer Planes is just a bit less than what they got in the 3e MoP, so that would be adequate for a reasonable overview.