I doubt it. I'm in the camp of thinking a Planescape book would also be a manual of the planes book (but not Spelljammer), and that means covering a ton of ground, from the Inner Planes to the Outer Planes and everything inbetween, including Sigil and the Outlands. I know people would prefer them kept up into two book (and they probably will skip the Feywild/Shadowfel) but I'm definitely thinking they're covering them all in one.
If they do cover all of that, I don't see how they could cover 12 factions with 6 pages per one. If they did feel like they need to print them all, I'd say they get more like 2 pages each, and more likely even less (2 factions per page).
Ravnica and Theros are unique for being pretty small in scope settings, so there is a lot of room to go deep with their gods and guilds. A Planescape book kind of needs to go wide, so I don't see how they can give the factions nearly as much detail.