VentureBeat have posted a Q&A with 
Paul Sams about Blizzard and game development, especially in the light of the Activision-Blizzard merger. He answers and comments on a number of questions, including the big question mark of the unannounced MMO. That question naturally gets no more than a comment, but interesting never the less:
PS: We have three announced games. You may be referring to the fact that some folks have noticed on our web site that we are hiring for a next-generation MMO. That’s listed on our web site but we aren’t ready to share anything on that yet. We’re hiring for that. We’re also trying to move to best-in-class customer service now.
Diablo or StarCraft MMO? Or possibly a new franchise? They could develop the MMO parallel to a current development, like with WarCraft III, in which case both Diablo and StarCraft are good bets. Both are getting a lot more complex lore, and both can end in a fashion as to make a transition to MMO easier, story-wise.
The rest of the Q&A session is most of the usual talk about game design from Blizzard. It’s interesting, but also something we have heard many times before.