"According to Winninger, the D&D Team is “particularly excited for [the slipcase] format,” and that we were “likely going to see several things in that format” in the coming years."
< Darth Vader NOOOOOOOO >
So we can expect more products with a laughably low page count, very little focus on the actual setting (rather literally most of it on monsters and an adventure you're unlikely to run more than once, if you even run it at all), and which are 40% to 80% more expensive than previous products, even those with massively more content?
Great. Sadly looking like my predictions of re-orienting D&D into a lifestyle/premium product may be true.
As for Planescape in that format, there would be just 64 pages to cover:
1) All player options.
2) The entire metaphysics of the setting.
3) The Factions
4) Sigil.
5) All of the planes (?!?)
So we could hard-guarantee no Factions, for sure. There's literally no way you could include them. So we'd have to have the ultra-dreadful Monte Cook "accident" take on Sigil, where he replaced all the Factions with a number of three-letter acronym organisations (with almost intentionally dull-seeming names) and gave Sigil the vibe of a sleepy and insular Midwestern city (not even a college town!). I say accident because Cook swears he intended to bring the Factions back and it was just mean ol' WotC stopping publishing 2E stuff that stopped him writing another adventure where that happened (Cook has form with thinking his setting ideas are "better" than those of actual original setting designers though so I am skeptical).
In the original 2E boxed set, by the way, the breakdown was:
Sigil and Beyond - 96 pages
DM's guide to the Planes - 64 pages
Player's guide to the Planes - 32 pages
Monstrous supplement - 32 pages
And the maps and DM's screen. There was no adventure.
If we moved this to the new format, that 192 pages of setting and player rules being compressed into 64. Insane, frankly.
Then we'd have 64 pages of bestiary, largely unnecessarily given most of the most important PS monsters are already in D&D, so it'd just be "odds and ends" and Modrons, who only need a few pages.
Then we'd have a 64 page adventure instead of actual setting or rules info.
Honestly that would be a crime, and a quite serious crime at that! If they flexed the design, dropped the adventure, and just had a short bestiary, though, I think that could probably work. But I doubt they will. I strongly suspect they'll stick to the format whether it makes any sense or not until they get pushback, either in people moaning publicly in sufficient numbers, or poor sales.
Greyhawk would make a lot more sense because it'd presumably have basically zero in the way of player options and compressing the core setting into 64 pages is a lot more doable.