Biel wouldn't be bad, but she doesn't scream "perfect" to me for the role. Although noone really does. It needs to be someone who the filmmakers can depend apon to bring in an audience so that it will be profitable. Here are some suggestions.
Natalie Portman:
Pros - Could easily pass for Russian, probably good enough actress to do accents well. Although established in the sci-fi world, it's for a character that's signifigantly different than a Russian spy/soldier type.
Cons - Doesn't really have the body that would best complement a leather bodysuit, could be type of actress who takes herself too seriously (ya never know)
Sarah Michelle Gellar:
Pro's - Has proven to be a successful transitioner from TV success to movie success. Probably most dependable actress in her age group when it comes to hand-to-hand combat action. Looks great in tight leather. The genre of a Black Widow movie's fans pretty much already like her.
Con's - Depsite success in all sorts of movies in between Buffy and now, both she and the filmmakers still might be concerned about typecasting. There might be this feeling that people are going to see Buffy talking Russian, instead of THE Black Widow.