^All this talk of Andrew Garfield reminds me of his Spider-Man
co-star Rhys Ifans. I think Ifans would be a great Doctor.
I don't know that Patrick Stewart would be viable in the long term. However, I have an idea for him playing an unspecified distant future Doctor in some multi-Doctor story.
While I think a female Doctor *might* work, I certainly don't think Emma Watson is nearly old enough to be able to pull it off. You'd need someone with a bit more gravitas, like Rachel Weisz. (I can just picture the Doctor regenerating into a woman. River expresses an interest. The Doctor shoots back, "Hey, hey, slow down! This is all brand new. I don't think I'm ready to start test driving everything yet.")