Now this is my kind of speculation post! I like a lot of what you speculate here. I don't have much to add, but I do have a few comments:
- I think they will touch on a lot of the groups you suggest. However, just as the have done in almost every other case, I think they will likely not cover all members of the group. So you will not get all 7 members of any one group.
- I definitely don't expect to get all the archdevils for similar reasons to point #1 and page count. Though we currently only have one of the "9," we also have two former members: Moloch & Geryon. I think we get 4-5 at most.
- I don't think we will get more demon lords, but I could be wrong.
- They have hinted that there will be more high level monsters in the '24 MM, so this could be an early preview of that approach.
- What about high level yugoloths? General of Gehenna anyone?!
- More inevitables?
- I don't think it will happen, but I would love to get mythic versions of the demon lords and archdevils (and just a CR upgrade to Demo and Orcus is needed as well).
I agree on a lot of this. However, I'm going to guess all the "traditional" archons and guardinals will appear. As I mentioned, there are only 6 original guardinal types, the most powerful of which (the leonals) are only in the mid-CR range (they were CR 12 in 3e). And I forgot that the tome archons, as rulers of the layers of Mt Celestia, were individualized in 2e, meaning both that they likely won't appear and that there are only 6 remaining types of original archons to worry about (although the throne archons are fairly high level, with a presumably larger stat block). They probably both collectively could fit into 4 - 5 pages each, which together would be no more than a sixth of our page count.
I agree, however, that the Archdevils and hierarch modrons, while important, are going be hard to fit in in their entirities, and we'll likely not see all of them, unless any left out end up in the likely D&D Beyond suppliment. Thinking further on this, giving stats for Levistus is pretty futile, as he's trapped in a block of impenetrable ice, making him an easy candidate for exclusion. I'd have to say that the more iconic Archdevils (which I would say would be Asmodeus, Baalzebul, Mephistopheles, and Dispater) will definitely be in, and we'll have to see about the others. As for the hierarch modrons, as they make a direct numerical sequence from 10 to 1 in their names, I guess if they do end up only doing some, they'll do the next lowest five, the decatons through the hextons. Granted, if they do, say, 2 pages of description and then two stat blocks per page, they could potentially get all 10 in just 7 pages, although art and a likely large Primus stat block would likely more realistically mean 8 or 9.
Some comparisons to the Spelljammer bestiary are very helpful here. There are a
lot of groups with 3 stat blocks, and they and the descriptive text all fit on 2 pages. The astral elves have 5 stat blocks, and their entire entry all fits on 3 pages. The two dragon types have some larger stat blocks, and fit 4 stat blocks and text on 4 pages, although in each case, one of the four pages is nothing but a full page illustration.
Hmm, maybe some time for math, and making estimates leaning towards a larger number of pages for groups, if they all hypothetically appear in the book:
9 (8) Archdevils: 18 (16) pages
10 hierarch modrons: 9 pages
6 archons: 5 pages
6 guardinals: 5 pages
3 demodands: 2 pages
That gives us 39 (or 37 without Levistus) pages out of 64, and assuming 4 pages used for the ToC and other introductory material, leaves us 11 pages for other things (13 if they skip out on Levistus). That's not excitingly good, but also not terrible, especially with the likely overflow room for 10 or so more stat blocks on D&D Beyond (which could very well include some of these - honestly, dumping some or most of the Archdevils there, where their larger page count is much less of a problem, would seem pretty reasonable)
