One thing I do think is important though, is that she doesn't play the doctor as
feminine. I don't think it matters whether The Doctor is male or female, for the character, but a major reason for that is that The Doctor is
not masculine. He's not some big gruff blokey bloke... if anything, he's almost "effeminate" (I know this is no longer a PC term
) in a lot of his mannerisms, especially Smith and Capaldi, but even Tennant and Eccleston. In fact it made the "Doctor Donna" scenes quite fascinating as she mirrored Tennant's Doctor-mannerisms really convincingly, without her seeming masculine or him feminine.
So long as they stick with that I don't think the biological gender will matter much at all. Despite those videos ranting about how The Doctor "is male!", he's not actually a very "male" character at all.