Faolyn
(she/her)
The PS books were filled with interesting details that, honestly, could be cut out without much of a problem. If we were to assume a single book that covered the planes, Sigil, and the factions (and using the layout of VGR as a guide):Like I said before, Theros doesn't need to cover nearly as much ground as Planescape does. If anything, the writers of Theros were struggling how to fill more pages with material. Planescape will have the opposite problem; what material can they put in the book, and what needs to be cut.
Each plane gets 3-5 pages. Planes like the Abyss would get more, although I personally would want a "random Abyssal plane generator" rather than list of named planes, since there are multiple lists of Abyssal planes available online. Each plane would include a couple of paragraphs on each layer, a "What People Know" section which includes examples of creatures and gods live there, as well as what factions, if any, have a headquarters there, and an expansion on the planar effects from the DMG--assuming that it doesn't just say "see the DMG for the planar effects. And then a sidebar for plot hooks. They seem to have removed the para- and quasi-elemental planes and shuffled them into the regular elemental planes. The positive and negative planes also seem to have disappeared. So, seventeen Outer Planes, four elemental planes, and maybe the Feywild and Shadowfell, so this section would be somewhere between 70-120 pages long.
A section on demiplanes, including examples and a guide for making your own. Probably another 3-5. pages.
A section on making your own cosmology, expanding what's already in the DMG. Probably another 2-5 pages.
A chapter on Sigil, with an overview and probably 1-2 pages for each of the six wards. Say, 15 pages total.
The factions, which would be a big thing. Probably 2-3 pages per faction, including advancement via renown, a la Ravnica.
Then races/heritages, archetypes, and monsters. Plus art.
As I said way earlier, I would much prefer if this were two books, one for the planes and one for Sigil, the factions, and the Gate Towns. But I think it's possible for them to squeeze everything in one book.