On the Warhammer Community page for the Las Vegas Open (here) where the aforementioned confirmation of each Grand Alliance getting at least one more battletome, the image attached shows a Vampire count. We can guess a Soulblight Battletome is in the future, hopefully a faction of Vampires too stuck-up to kneel before Nagash. This, if true, would explain why Vampire Knights and Zombies didn't get new miniatures with the Legions of Nagash release.