I would be very surprised if it were not a StarCraft MMO, which I've been saying for years. I can't think of a better property for Blizzard to use at this point from a business perspective for a few reasons.

1. There's an overwhelming demand from fans for a StarCraft MMO.

2. It wouldn't detract from WoW. It's a sci-fi game that would have a completely different setting from WoW's fantasy theme. Sure, there will be people who leave WoW for it. But it's all money in Blizzard's bank account either way.

3. There aren't any AAA quality sci-fi MMO's out right now. The market is up for grabs.

4. SC2's updated graphics can set the stage for and provide a smooth and easy transition into a 3D StarCraft world. The graphic style that you see today in WoW is almost identical to that of WC3's. It's a familiar style of graphics and art to people who have been playing WC3 for so long. Blizzard obviously knew people loved that style so much with the tremendous success of WC3 and hence they intentionally adopted it for WoW. SC2 is part of a much bigger plan other than just serving as a great sequel to a great game.