Keira Knightley may seem like a bit of a laughable choice. Think about it. She's not exactly known for being rough and tumble. The last time she seemed tough at all in a film was 'Bend It Like Beckham.' She's a fantastic actress, especially when in period pieces, but her action star qualities are questionable. She was in the movie 'Domino' about a female bounty hunter, and the movie is actually pretty good, but she still seems like a stiff wind could knock her down.
Her star power and acting skill could be enough to land her the role, and it's not as if she's without action movie merits. As I mentioned, she was in the movie 'Domino,' and there's also her work in 'Pirates of the Caribbean' which has its fair share of action.