My answer may be swayed by the fact that I love Audrey Hepburn. It's between her and Summer Glau over who is my favourite actress.
Still, I think she could play a kickass character well. To me she often portrayed a lot of inner stregnth. I think she'd make a good Whedon herione, someone so dainty who you don't expect to be tough turning out to be really strong. Still, like I said, I may be being swayed by how much I love Audrey Hepburn.

Basically, because Katherine already kind of was a female ass-kicker of her time. She was unconventional, often wore pants, and took interesting roles.

Second choice might go to Vivien Leigh, based on her performance in Gone with the Wind.

Yeah, I agreee with Cinders!

Oh, yeah. She can do sexy, vicious, manipulative, and/or likeable, zesty, she's got the package.

She played Cleopatra. Enough said.