As much as everyone would like a Lost Vikings MMO, or even a World of Warcraft 2, or a long-rumored Universe of Starcraft, I'm going with the last answer: none of the above. Blizzard could adapt any of their worlds to the massively format (well, except for Rock N' Roll Racing, but Massively has included it anyway), but considering that Starcraft 2 and Diablo 3 are already in the works as non-MMOs, my guess is that Blizzard is going to break new ground with a brand-new IP whenever they return to the MMO format with another game.
Of course, this being Blizzard, they're not likely to give any hints until they want to give them to us, and if for some reason the next-gen MMO never does come about, we'll probably never even know what it was. But speculation is half the fun. What would a Blackthorne MMO even be like -- maybe a free-to-play sidescroller?