Why bother guessing? It's Starcraft.
Blizzard representative Drysc left a small comment in the World of Warcraft forums which indicates that the company is indeed working on a brand new MMOG instead of just a WoW expansion.
No, it is an unannounced Next-Gen MMO.
And that doesn't mean an expansion for World of Warcraft either.
Take a look at the original thread.