Giants have been stated to be next, with allusions to Shakespeare. I'm guessing big fat shenanigans in the North, where all the Giants and Humanoids are (Orcs, Goblins, Ogres, etc allowing the rampup to full Giants), maybe an implementation of the mass combat rules from AU.
Past that:
-Fey of some sort, battle between the courts with the players caught up in the middle. Good opportunity to throw in new PC options in addition to monsters.
-Shadowfell (terrible name), probably drawing in Ravenloft and other iconic stuff, as others said up thread. Good character option potential as well.
- Far Realm: Lovecraft corpus is public domain. Dagon and Cthulu, fully stayed. Psionic rules, including class options and races (Mearla said they were not including Kalashtar in he Eberron article because they would be treating Psionics elsewhere).
- Lost world hex crawl, namely the now cosmic Isle of Dread (DMG has Isle of Dread as floating between different prime material realms via the Elemental plane of Water). So much potential here.
- Devils: they are different from Demons, so different stories present themselves here. Maybe we'll get a Iuz focused AP when they do Greyhawk, that draws in Asmodeus and company? Cultists optional...
- Riff on the Great Modron March, or maybe some sort of Modron-Slaad conflict. Plenty of seeds in the MM and DMH for this, maybe they could get Monte Cook involved...
-Gotterdamurung (sp?), war in the heavens draws players into the conflict. Kooky D&D polytheism leaves plenty of room for this.
- Atlantis Attacks, all the underwater civilization stuff gets thrown together in a "fish out of water" story starring your party. Also lots of hooks in the MM for this, but plenty of room for them to develop more.
- Culturally themed, like say an Egyptian style AP set in Mulhollerand (sp, phone) which is apparently a thing again, that can be ser in some other Egypt analogue as desired. Use all the mythos specific monsters, introduce new ones.
Some thoughts based on the core books, at any rate.