We've been hearing a bit on the progress of Thor 2 but nothing concerning villains or new characters. It's been reported that we could see the cast of Asgardians double. So that means we could see at least six new characters in the sequel. One character that's been knocked around since Iron Man 2 is Scarlet Witch.
Yes, in the comics she is a mutant and the daughter of Magneto. However, she's been more active in The Avengers and West Coast Avengers comics and has magic based powers. This might exclude her from the X-Men/Fox deal and that she very well show up in Marvel Studios' films. Previously director Jon Favreau had Scarlett Johansson in mind for the part when Witch was originally included in Iron Man 2. Emily Blunt vacated the Black Widow role which was filled by Johansson leading to the character to be cut from the film. I don't think Kevin Feige would have even allowed Jon to play with the idea unless they actually have control of the character. I could see the studio making Witch an Asgardian since it's a good way to reinvent the character for the screen.
It wouldn't be the first time Marvel has reinvented a character to fit their Cinematic Universe. Bucky Barnes, Black Widow, Whiplash, Justin Hammer, Jane Foster and Thor have all had their characters retooled to fit the films so it wouldn't be shocking to see it happen again. My thought is that Scarlet could end up being one of the many new Asgardians in Thor 2. Thor's rogues gallery is full and colorful his group of cohorts is somewhat unbalanced. The amount of villains out weights the group of allies and friends, considering the scale of this fantasy world. We should expect characters like Balder (The Brave), Amora (The Enchantress), Skurge (The Executioner) and Brunhilde (Valkyrie) but the impression I get is that we could see the studio give us even more. Scarlet is also a prolific member of the Avengers so she could make an excellent addition to the mostly male lineup.