M.L. Martin
Hero
The preview in The Ghost King involves Tharizdun, the Princes of Elemental Evil, and their cults . . . along with another entity, called "The Progenitor" who's in communication with Tharizdun.
Do they mean something from 1E or 2E that was there and since disappeared from the game largely? That would be kind of awesome.
Tharizdun most likely.
I remember that Gygax wrote a bunch of Greyhawk novels that involved a lot to do with Tharizdun. It was the Gord the Rogue series, and it involved many things that never made it into D&D, and dealt with the end of the Multiverse that Tharizdun would cause.
While it won't be a retelling of that series, the Abyssal Plague might involve Tharizdun, simply because he has a lot to do with the Abyss in 4e.
The Queen of Chaos might not be quite shocking enough given that they hinted towards her return in Demonimicon.
Maybe He Who Was is coming back to even the score with his former servent.