3. Emily Blunt
This couldn't be more of an obvious pick, and I'm really not ashamed of that. Beyond the requisite "proper British intellectual" background for the character, Blunt also happens to be gorgeous, has a diverse acting range, and proved she can become a huge action star. Not only that, she has the distinct advantage over my other candidates of being a full-fledged A-list movie star with enough recognition to headline a franchise.
Witness her turn in Edge of Tomorrow (aka Live.Die.Repeat.) as a hardcore super-soldier teaching Tom Cruise how to not die so much. That's the action star we need. But also check out the level of drained desperation she exudes through the second half of Looper. The character's been emotionally beaten down the same way Lara must be before... you know, rising.