Additionally, I am not too optimistic on a StarCraft II strategy title, mostly because huge parts of the SC team left Blizzard after it was completed. I bet they will release a sequel, but perhaps not as a strategy game type.
If I happen to be wrong, there's only one thing I could think of that would stop World of StarCraft: the fact that it may split Blizzard's market in two pieces instead of doubling it. As for Diablo, this world is actually far too similar to WoW. It's all ancient stuff, wizardry and monsters. So maybe Blizzard wants to stick to what it has done before.
Anyway, selling a game for 50 bucks and then milking its customers on monthly fees is probably Blizzard's new thing. Imagine how much more money they make on WoW than on SC and Diablo.
I would personally want a sequel to StarCraft out sooner than Diablo, while I still remember the superb story that it and Brood War offered.