there are some spoilers here. so be warned. a Marvel employee (i have yet to find info on his name). has realveled the plot for thor 2.
"Thor becomes reunited with Jane who becomes possessed by a Dark Spirit, which is connected with the Dark Elves; a race the Asgardians have fallen out with many years beforehand [...] The Dark Elves attack Asgard, and they take a huge hit. So Thor has to rebuild Asgard by putting together an army to go and take the battle to the Dark Elves and defeat them. But the only way he can do this is – he needs to cross over to the Dark World, and the only person who has access to the dark side of Asgard is his old brother Loki, who at the beginning of the film is locked up in prison. So what happens is Thor basically has to negotiate this truce with Loki, and he goes on this great trek with Jane, all three of them. They go to this far side of Asgard, which is being shot in Iceland, where they train and assemble an army to go and attack the Dark Elves’ world. the exclusive twist…is that one of the things that happens during the Asgardian attack is that Thor’s mother gets killed. So it becomes like a vengeance thing. And they were keeping very tight-lipped about what actually happens with Loki, you don’t know whether he redeems himself or he’s still the bad egg."