Well, apart from the obvious choice of buying the rights to make The Sims: Diablo's Dollhouse,
there's only one thing blizz could really be doing, and that is a brand new project.
It won't be sc2, as the two teams who originally made sc (back when they were one team) are making wow and still working on war3 patches and probably started working a bit on the exp. It won't be war3exp, it is waaaaay to soon for them to even consider announcing that (even though it's definitely going to be the next thing the war3 team do).
So all that's left is teh blizzard north team, who made the diablo, and my guess is, they don't want make another diablo game after doing effectively 3 in a row, so that just leaves something brand, spanking new!