Starcraft map and Starcraft hero would be really great. Although hasn't Blizzard said that they don't want to do maps that are too tied to their existing franchises? Or was that just the plan for the original maps?

Also, technically, Mengsk is an excellent fighter, he trained for it all his live. Though I admit that he's probably better as a buffer/debuffer, a bit like Abathur, only more active.

They could also add a hybrid hero, though I suppose that would be basically reskinned Diablo, kit-wise.