What we know from Blizzard employees about Blizzard's upcoming MMO

1) Blizzard's new MMO will be a new franchise

Community manager Zarhym posted to the World of Warcraft forums:

We have nothing to advertise for the new MMO because it's a shell of a game thus far. We've already stated it'll be a brand new franchise, which means the lore, art, and game play are being developed entirely from scratch. It's an overwhelming process, but a process through which we excel. Our track record supports this.

2) It will have broader audience appeal than previous Blizzard games

Bobby Kotick, CEO of Activision Blizzard, said at the Deutsche Bank Securities Technology Conference:

What we've announced at Blizzard is that we have Starcraft [II] coming, Diablo has been in production...and a new, unannounced MMOG that has a little more broad appeal

3) It will probably contain some science fiction and/or fantasy elements

Jeffrey Kaplan, currently lead designer for the Blizzard's next gen MMO, when asked if the game would be science fiction, post-apocalyptic, or historical, laughed and responded: "all of those combined!"

Several Blizzard job listings lend more support, for instance:

2+ years game development experience modeling and texturing sci-fi and fantasy character armor and weapons for next-gen art pipelines

4) It might be on consoles in addition to PC
Jeffrey Kaplan, in an interview with MTV Multiplayer, hints that Blizzard might adapt a future MMO to consoles

So I don't think 'World of Warcraft' would really work that well on the console, but that's not to say that other MMOs couldn't. And we have a pretty savvy group of console developers -- a lot of the guys we inherited from Swingin' Ape [Studios] really know what they're doing on next-gen consoles. So we might have a few tricks up our sleeves in that regard.