There's no more evidence for Starcraft 2 than there is for Diablo 3 or any other game.

But I can tell you this. Blizzard will not release another MMORPG this soon. They would be stepping on their own toes, and that's never good. They could do a starcraft MMORTS or something along those lines, but World of Starcraft would be retarded for them to do as an MMORPG. Not that I'm saying the IDEA would be bad, it would probably be a sweet game, but WoW players would have to decide between playing WoW or WoS, and most of them would probably choose the newer game. WoW would effectively die, and blizzard would simply shift it's customers.

My guess is, they'll have a new RTS(Starcraft 2), a single and small scale multiplayer game(Diablo 3, not as an MMORPG), or they will release a completely new game from a completely new series.

It could just be a new expansion for WoW, but that's stupid. It's hard to say what it is, anyway, because there's no evidence toward any of it.