Older Beholder
Hero
Unlikely, but a Creatures of the Underdark could be interesting, although I'm not sure there's that much left that hasn't already been covered in detail.
Given that they're bringing back Planescape, perhaps something along the lines of "Skall's Book of the Dead"?My bet would be for a Liliana's Libris Mortis, yes, even if Liliana is a planewalker from M:tG, but today the limits between D&D and M:tG are more and more diffused. Fistandantilus from Dragonlance was a relatively famous necromancer, but now "he is not in this neighbour". Gilidarius, Raistlin's former apprentice is practically unknown. Or maybe a Ez d'Avenir's guide of undead.
Vecna's Scriptorium of Shadows?Given that they're bringing back Planescape, perhaps something along the lines of "Skall's Book of the Dead"?
Vecna's Crypt of the DeadVecna's Scriptorium of Shadows?
I imagine that would inevitably be "Iggwilv's Demonomicon".Somebody (I can't think of, sorry) Guide to the Abyss
You could bring back Primodials in a big way with an elementao book too.Excellent point on elementals. There's plenty of room for bestiary additions (genie and elemental variants, para-elementals as you said, maybe even the quasi-elementals and energons), and lots of possible player options/info (genasi info, elemental subclasses as you said, and potential warlock patrons). Lots of possibilities for magic items and spells as well. And it could even double as a sourcebook for the Inner Planes, assuming that Planescape more or less ignores them as it has in the past.