Older Beholder
Hero
If every card in the deck gets 10-12 pages of material (and so far what they have said suggests Subclasses, Feats, Spells, Monsters, and maps), that's a full length book. Nothing so far suggests a Xanathar's/Tasha's style release.
I did think for a while that it could be another Xanathars/Tasha’s, probably just because of the ‘many things’ in the title. But this doesn’t fit with the previous convention of having a famous characters name.
But it’s pretty speculative at the moment. I think The Book of Many Things is the product I have the least idea what it will actually include.