Unannounced Blizzard product not a Starcraft, Diablo sequel

When Blizzard announced that its northern offices were being relocated to California, they announced that they were working on a new, unannounced
project that was believed to be Diablo III.  Well,
Blizzard's PR people have quickly squashed that rumor, stating that the new project is neither a sequel to Starcraft or Diablo.  So now the new speculation is that this project is an expansion to World of Warcraft, but it could be a new franchise all its own.  We don't have to wait long to find out, though: the new game will be announced at their inaugural BlizzCon in October.