I really hope it's Diablo 3. Of course anything made by Blizzard is great, but please don't make some MMORPG, though I think they know that two MMORPG's at once would be a disaster.
Of course they won't make two MMOs. I am willing to bet that a Diablo MMO would become more popular than WoW.
i'm pretty sure another mmo by blizzard is in the works. take a look at their job listing 6/16/08 and 6/17/08: http://www.blizzard.com/us/jobopp/
this doesn't mean much though since it might be one of the several future expansion packs of WoW, but the job title does specify next gen.