Dark Fantasy
Diablo fits into this category, but so do many other games and settings. Everything from BioWare's upcoming Dragon Age -- which leans a little more towards classic fantasy with elves and dwarfs -- to the aforementioned Warhamchine, which while heavily steampunk also carries many dark fantasy connotations.
It's hard to guess what's going to be popular in a couple years, let alone the several it takes to develop an MMO. Still, one thing you can count on is for people to be interested in a dark world with a mysterious backstory. That's why I'm fairly certain that whether it's science fiction, fantasy, steampunk or whatever, it'll probably be darker than World of Warcraft.
Science Fantasy
It's not a term you hear too often, but Science Fantasy is something everyone probably quite familiar with: Star Wars. This is basically a world where, while science fiction is seemingly the source setting, fantasy is always lurking somewhere in the background. Star Wars being the very obvious example with the Force, evil emperors and Jedi, but there are other settings as well.
A more recent franchise would be Mass Effect, which leans more towards sci-fi, but still carries some classic fantasy trappings like the game's reliance on dark energy to explain why people can perform the equivalence force push and create gravity wells. Still, Mass Effect is largely a sci-fi game first -- but the example holds.
Obviously this would be double edged choice with Star Wars: The Old Republic coming along in a few years. Could Blizzard out-Star Wars, Star Wars? That's a pretty tall order and something I don't even really know how to answer. Part of me just wants to see them try it, to be honest.
Conclusion
Whatever it is that Blizzard is cooking up you can bet that it'll be their take on an established genre. Will it be a mixture of one or more of the above? Possibly, especially since I did cover a decent spread.
There's always the chance that we'll all be pleasantly surprised by something completely unexpected. I would say that with two major fantasy heavy titles and only one sci-fi title under their belt, chance favors this new IP to be sci-fi focused. On the other hand, Blizzard can kind of do whatever they want, It's really up to the men and women in charge. Something tells me that whatever they do, it'll be worth playing regardless.
