Boy, is this definitely a case of "he said, he said." Not too long ago, Doctor Who showrunner Steven Moffat was teasing us that the next Doctor could be a Time Lady, and now the Eleventh Doctor himself, Matt Smith, is also chiming in on the topic—but his tune is a bit different.
In an interview with The Mary Sue, Matt Smith discussed the topic of having the Doctor transforming into a Time Lady during his next regeneration, and who he'd like to see in the role, were it ever to happen.
I think there are many actresses that could play it because there are so many wonderful actresses. I mean, it would change the role because she would be a woman so when you put her in a room full of men, it's a different scenario than if you put a man in a room full of men, because she's a different sex. Would it change the fundamentals of the character? No. But it's an interesting idea, if the Doctor's a woman, does she have a Doctor baby? Is there a mini-Doctor? I don't know, who knows? What actresses could play her? Oh gosh, so many, so many could play her. It depends on what age you would want. [He was really thinking hard here] It could be...Charlize Theron is pretty kick ass, isn't she? They just have to find a brilliant actress. I never see that happening any time soon to be honest. I don't think it will happen. [Because you're not leaving us?] No. And I'm not turning into a woman.
There you have it. Matt Smith doesn't see it ever happening—unlike his boss Steven Moffat—but if it did, he'd love to have Charlize Theron in the role of the Doctor!
Pretty interesting choice, don't you think?