My guess is the big announcement will be something to do with Diablo3 for 2011. Just like two years ago, be prepared for something lackluster. If they announce the new MMO it might steal some thunder from Cataclysm, so while it would be cool to hear about it I doubt they will do that with only a couple weeks before a major launch.
Of course they migh have some news about the second part of SC2 for the end of 2011 (game engine is done, so it should be just story and cinematics)