If I got everything right, there will be three more "Endtimes" books, one of them having a Chaos focus (the one for October, with the "big and nurgly" thing), the other one with Skaven in them.
The Chaos one will most likely feature Archaon in a prominent role - he is the main antagonist of Nagash after all. I think the Empire will play a major role in this as well, and possibly Dwarfs, Ogres and maybe even Chaos Dwarfs.
The Skaven one - and that is pure speculation on my part - could be "Endtimes: The Underworld". Skaven, Nightgoblins (plus other things, to keep the general O&G players in as well) and of course a big spotlight on Dwarfs.
The remaining third book? Elves vs. Daemons on Ulthuan, with Lizardmen battling Daemons on a "second front" in Lustria.