I'm suspecting there's a major superhero prison break going on. The first time we meet the twins they are in separate cells, trying to get to each other and convince everyone they're ready to go straight.
It's also a fairly good way to bring all the Avengers back in as well as opposed to the twins already attacking a city and suddenly the Avengers turn up fully formed.
Then again, it doesn't really make sense to bring a guy like Thor in for a prison break. He would nuke the entire prison. I'm sure Joss would make it work.
I'm so excited to see the twins!