Katee Sackhoff or Michelle Rodriguez. Right now, they're the only mainstream actresses with the right body build and the attitude. Only problem, the latter can't act and the former's too happy.
The problem with Shepard is that he's/she's a Big Damn Hero type, and there are only a limited supply of actors that can play that part. Most of them are aging (Bruce Willis, Bruce Campbell, Nathan Fillion) and there aren't any real young contenders for that spot. Jason Statham's funny but entirely wrong. Ok, as a compromise, we'll stick Joseph Gordon-Levitt in because I'm convinced he can play any role, *any role*, he likes and do well.