So, I'm going to start speculation on the one part of this set we can actually have some reasonably concrete ideas on what might be in it: the bestiary. Since we have an equivalent 64-page bestiary from the Spelljammer set, and we can check out old Planescape and other extra-planar products from previous editions to see what prominent creatures have yet to be updated to 5e, we can already get some ideas on what creatures might be included in this set.
Boo's Astral Menagerie is 64 pages long, and contains (if I counted correctly) 72 stat blocks. Most pages have 1 or 2 stat blocks per page, with one page each having 3 and 4. No single creature takes up more than one page, unless you count full-page splash art like for the dragons. So this would seem to be a good baseline for what might be in the Planescape bestiary.
The real question is what extra-planar creatures have yet to be updated to 5e, how much room would they take, and which ones are important enough to update and which ones might have to be left behind.
Firstly, given the art already released, and just from iconic importance to the setting, the dabus will 100% be in this bestiary. I'm almost as certain, again since they are iconic, that bariaur will as well - if they aren't outright a player choice! Also, given tradition and how much fans of the setting would go into absolute revolt if it happened, I'm going to wager that the Lady of Pain will definitely not appear in the bestiary (well, not given stats at least; she might make appearances otherwise).
There are a good number of groups of creatures that still need to be updated. Groups are good - they can be efficiently packed into a smaller number of pages due to having a normal length entry for the group as a whole, short descriptions of individual members, and the rest of the room filled with the actual stat blocks. However, the first group is just the opposite of this and could definitely cause issues with the available space...
The Archdevils. While the big name Demon Princes have been described and reprinted in multiple products, we've only had some of the lesser Archdevils appear so far, with only a single one of the Lords of the Nine (Zariel). This definitely would seem to be an ideal product to update the remaining major Archdevils, especially as they have always been a favorite subject in pretty much every edition. But... they pose a problem in a limited page count like we have here: going off the examples of the Demon Princes and Zariel, they will take up two pages each. Yikes. As we are missing nine remaining rulers of the layers of the Nine Hells (yes, we have Zariel already, but Phlegethos is co-ruled by Belial and Fierna - a fact confirmed by the DMG - that brings us back up to nine), that's 18 of our precious 64 pages already taken up. But... I can't see how they can be skipped, or at least not all of them. They're just too iconic, and it's surprising that they haven't been updated already. Maybe they'll cut down on the lore sections to fit them into, say, 1.5 pages each?
Remaining Demon Princes. The most iconic Demon Princes, as stated before, have already been updated, but there are a few fairly major ones that haven't. The most notable of these is Pazuzu, who controls the skies above the 1st layer of the Abyss, and who's been a mainstay since 1st edition. Kostchtchie would be second on this list, but he and his stat block appeared in Descent to Avernus, so he's could easily be skipped over. Beyond that, there are some fairly iconic Demon Princes still left, such as Dagon, but really, with the truly big names already done, they'll probably just be ignored, other than Pazuzu, especially since, again, they could take up 2 pages each.
The Demodands/Gehreleths. Iconic since 1st edition? Fiendish foes that characters could very reasonably be opposed to and fight? And there are only three types so they won't take up much room in the book? I'm pretty certain they'll be in.
The Hierarch Modrons. The current WotC team seem to love modrons. So it's odd that we've only seen the five lowest-level members of the type. Granted, the higher-level hierarch modrons rarely leave Mechanus, so their non-appearance does make some sense. However, they are seen around the Planes, especially during the Great Modron March. So we could reasonably expect to see them in this book. The only difficulty is that there are 10 types, which, as we get to more powerful ones, will likely have bigger stat blocks. Primus, the modron at the top of the hierarchy, will almost certainly take up a full page (though probably not 2 full pages like the Archdevils/Demon Princes). So this could potentially eat up yet another substantial portion of the book's real estate. But, again, it's hard to see them not getting in.
(And if they do make it in, please have clear art for each type! It's really hard to find clear official art for several types of the hierarchs outside the cartoonish ones from 1e. Just Google images for "nonaton" - are they shaped like a group of spheres arranged like a centipede or are they cylindrical-ish?)
The Celestials. Celestials are one of the most neglected groups in 5e, with very few creatures of the types having appeared in the edition so far. This would seem to be the place to rectify that, especially as Planescape's ethos means that even good adventures might run afoul of them. And, given that ardlings, who are descendants of celestials, appear in the new revision's playtest, it seems to be a good time to flesh out their ancestors. Firstly, there are the LG archons, who have been around since 1e (outside of their name being used by entirely different creatures in 4e). There are 7 traditional types of these (there were later additions, but given our page count, they'll probably ignore them to cut the bloat); since several are low level, they likely will be able to packed fairly efficiently. I'm going to say they will almost certainly be in. Next are the NG guardinals, who, as anthropomorphic animals, are the easiest to envision as ancestors of the ardlings. There are 6 traditional types of guardinals, all relatively low level, so, they too can probably be packed fairly efficiently in just a few pages. Again, I'll say they'll pretty likely be in. Finally, there's the current problem child of the celestials, the CG ones that originally used the name "eladrin", which, while having similarities, are still notably different from the group that appeared in 4e and now bear the name. Will they appear, but with a different name? It seems ripe for all sorts of confusion, but it would be odd if the CG corner of the Great Wheel is missing an iconic race while the others maintain theirs. If they appear, there are 7 traditional types, and, again could probably be packed fairly efficiently. However, of all three groups of celestials, I would say they are the most likely to not make the transition.
The Rilmani. Of all the big groups, this is the one I think is most likely to be skipped over. They only appear late in the Planescape books, weren't really updated in 3e and beyond like the others, and of all the iconic alignment-based groups, as the true neutral representatives, they're rather bland, being mainly known for their reticence and reserve. Hardly things to base grand adventures around. There are 7 types should they appear.
Remaining Fiends. Although the MM and MotM have managed to get almost all the fiends that could be considered traditional (i.e. those that just weren't weird one-offs in their product appearances), there are a few that have still slipped few. Most notable of these is the piscaloth yugoloth, which has been around in 1e and most editions afterwards, but somehow failed to make the cut for MotM. There are also some other devils and demons who have some minor traction through the editions, who might get updated here as well.
Formians. Since 3e shunned the modrons, it used the ant-like formians for their LN extra-planar creatures. Given their use there, and appearance in other editions, I can definitely see a case for them. Planes of Law lists four types (worker, warrior, myrmarch, and queen), which would be a reasonable number to have in any update, and easily packable in just a couple of pages.
Others. There are of course plenty of other Planescape creatures that could be updated - it just depends on what is chosen out of the bunch. A lot of them were in 1e and/or made the transition to 3e, and those would seem to be the most likely to continue on into this editions. These would include, for example, achaierai, animal lords, bebiliths, bladelings, hordlings, and vaporighu.
One further thing to consider: both Spelljammer and Dragonlance had extra creatures released for free on D&D Beyond - 10 for the former and 9 for the latter. I would think it's almost certain that we'll see something similar released for Planescape creatures that didn't quite make the print cut.
So - we have 64 pages (and a likely D&D Beyond appendix) to get as many of these in there. Which ones should make the cut? Which ones should be left out? Which ones did I miss that should be in?
