With the announcement set for the 15th of May, in S. Korea, where there are tons of internet cafes and gaming places, I would think it was Starcraft 2, myself.
WoW 2? Doubt it. With the successful launch of Burning Crusade, there's no need to release a new version of WoW, just expansion packs (hell, BC sold 2.4 million in ONE day).
Diablo 3 would be a surprise. There is still a big D2 base, so it's not totally out of the question.
But I still think it'll be a new Starcraft game. Either SC2 or Starcraft: Something...