Man people's sense of humor is lacking, would pick some out but I don't want to call anyone out.
Something WoW related but over the top at the same time... lets think...
-Anything mentioned pre- 04/1/13 is excluded. When Blizzard does something it's "brand new". So all the people saying "Hearthstone" (whether your saying it cause you actually think its going to be an April fools joke or you are beating around the bush to say you think the game is a joke) are wrong.
-Probably not something done in the past already (new classes, etc.)
-Something players might *think* they want but Blizzard presented the idea in an unwanting way. (Example: New character models but they look terribly bad or something)
Personally I have no idea what Blizzard will pull out of their sleeves. But I'm sure Crabby will make a return. Anything else is a gamble.