They dropped a teaser trailer for Thor: The Dark World recently, and although brief, it looks pretty dark entertaining.
It appears that the villain will be Malekith the Accursed. He’s a dark elf who also happens to be one of the few sorcerers with the power and cunning to Rival Loki, as well as being the former ruler of the Dark Elves. It’s unclear if he’s allying with something else in the movie, or if those are Dark Elf war machines or something. IIRC, the Dark Elves weren’t particularly evil in the comics… except Malekith. Even his own allies, which included some real bastards, hated him.
There is, however, one line I absolutely want to see in either this movie or a sequel:
Odin: For Asgard!
Thor: For Midgard!
Loki: For myself! 