The Master and the General changed gender, though. In fact, the General had only been male for one incarnation, but the rest of the time he has been female, as dialogue indicated.
All I just wish they do if the Doctor becomes a woman, then I want the change to be a lot more painfull as usual as he is not only changing cells, but changing gender as well.
Or to say it in the words of Capaldi: "We can only hope".
I think there are some actresses who might pull it off. Tilda Swinton is definitly one of the women I can see playing the part.
But as mentioned by Ranting Thespian, they shouldn't cast a woman for the sake of casting a woman. Just build a good script around it.