The fact that an actress is a woman is only one of her attributes. It's not the single, exclusive thing that defines her. Anyone who is cast as the Doctor is cast because they have qualities that make them ideal for the role. The same would be true of a female Doctor. Her sex would be one element under consideration, but most of the reason for casting her would be her personality, just as it is for a male Doctor.
I think realistically if there is a female Doctor it will be because they are specifically looking for one. I don't think casting would be like an open "Britain's Next Doctor" competition show or whatever and Susan Boyle ends up in the TARDIS.
If they could get someone like Tilda Swinton to play the Doctor and could write it well I would probably be happy. I just don't think they should feel a need to get a woman because of some sense of obligation and half-ass it because "the time has come".
This poor thread is never going to get back to Capaldi talking to the kids, is it? Sorry...