While she is already in Thor, I think Jamie Alexander would be good for WW:
She has the look, the age, and the action/acting chops (from what I can tell). While I normally don't like actors being in multiple comic franchises, I would be fine with this as she is not a major character in Thor, nor is she actually a superhero. Or maybe I just want to see her in more roles.
As for Beckinsale, I like her a lot, she is unbelievably gorgeous, and I think she can really act, but I don't think she is right for WW.