They need another one too. And, in doing so, the rest of Earth's cash-money. Since they've been advertising for ages on their site for more creative staff, everyone was kinda expecting it, but Gamespot managed to confirm with a Blizzard rep that the new project wasn't an add-on pack, but a new next-gen MMO.
"No, it is an unannounced next-gen MMO," said the rep. "And that doesn't mean an expansion for World of Warcraft either."
See! Told you. Which explains why they were so cagey when Jim went to see Starcraft 2 recently, which reminds me when I visited Blizzard circa the time of Diablo II when they mentioned that I couldn't go down a passage, as their other project was in that area. The urge to shake off my handlers and run, screaming (and probably naked) down the corridor to get a world exclusive was almost unbearable.
Anyway - New Blizzard MMO. Whatever could it be? Speculation-brains are GO!