MonsterEnvy
Legend
I think it would fine in a three-book slipcase so long as they cram in more. All three books should be 94 pages.Speaking for myself, I think doing Planescape in a three-book slipcase format can work out alright as long as they don't try to cram in too much.
I would adore a proper, full-sized hardcover Manual of the Planes for the Great Wheel, but that probably would be too much without a drastically increased page count or dedicating a fourth book in the set to it - maybe we can get one down the road at some point, but for the time being it's probably best to let that aspect be carried by the existing planar entries in the DMG, whatever specific planar locations are detailed in the adventure module(s), and previous edition/3rd party material on the DMs Guild.
Instead, they should probably just focus on a solid city-guide for Sigil in the player-facing book (plus the standard player options, faction descriptions, etc.). If they do that and make a decent plane-hopping "Tour around the Great Wheel" for the adventure book, I think that will work well enough.
Particularly given that this release should also finally open Planescape up on DMs Guild.
My idea is Book 1 Sigil, Book 2, the Planes and some creatures in each of them, book 3 adventure.