A very interesting interview on
Onlinewelten with Rob Pardo, the Executive Vice President of Blizzard Game Design, confirms that there is a fourth development team working on an unannounced, "unknown" Next Gen MMO. Speculation as to what it will be ranges from a Starcraft MMO to an entirely new Blizzard franchise. An interview with Paul Sams on
Gamasutra addresses speculations of a new franchise, saying Blizzard will release a new franchise sometime.
Exerpt from the Rob Pardo interview:
Official Blizzard Quote:

Interviewer: A few months ago you offered a job on the Official Blizzard web page, concerning a next-gen MMO. A community manager added that its an unannounced title. So, is there any new information on that, or is Diablo 3 the mentioned Next-Gen MMO?
Rob Pardo: No, its obviously not, since Diablo 3 isn't an MMO. So we have another development team...
Interviewer: So there is still one project...
Rob Pardo: Yeah, there is still an "unknown" project...
Excerpt from Paul Sams Interview:
Official Blizzard Quote:

Blizzard has very strong IP, but it also seems like all of it was created in the last century.
PS: First of all, that’s true that we’ve not introduced any new franchises this century. I think what it really comes down to is, we ask the teams what they want to make next.
I think the folks at our company are concerned, if they’re going to do a new franchise, that it be worthy – and that it be something that is equivalent or better than what we’ve done before, because they don’t want to do a franchise that’s not.
These are three of the best-selling franchises of all time, and very well-loved. So you’ve got a pretty high bar of trying to figure out what those franchises might be.
What I will say, though, is that the idea of a new franchise is very intriguing to employees of the company. Are we ever going to release a new one? I would absolutely say we will at some point. I just don’t know when that day will be quite yet.