The next question, is, why? Why should they do this? Because it would be a very, very good idea, for the producers of the most popular currently-running science fiction show in the world, to have a female protagonist. To have a female be intelligent, wacky, condescending, sentimental, courageous, hot-tempered, kind, complex....all the characteristics they've given to men over the last 50 years of the show.
That is to presume there would be some inherent advantage to a female protagonist (which you say, as if an unheard of thing in modern media). There wouldn't be. There would be no difference for a female to play The Doctor than a male to play The Doctor. It would be pointless to make him a her based simply on the idea that "they've never cast a woman before". Well, they've never cast a head of cabbage as a companion, so does that mean they should? (No, Tom Baker, no they shouldn't...)
And get this - she won't need a man to help her. Maybe she should have a male companion who's always getting into trouble and screaming for help, and SHE would have to rescue HIM all the time. Or maybe she should have a male companion who falls in love with her, and she never gives him the time of day, like they did with the Doctor and Martha. Just switch it around a bit. Maybe, gentlemen, you'll finally be able to see how annoying and offensive the show is and has been to women for most of its history.

I believe you are living about twenty-five years in the past, my friend. Doctor Who is not the chauvinistic show it was back in the 1960s and 70's. I know it's fun for people with "causes" to try and perpetuate that myth. But, I believe if you do some homework, you will find that there is a HUGE percentage of women that make up Doctor Who's audience. And I would guess that a majority of them would hate to see The Doctor as a female. One, they would no longer have some sexy piece of Time Lord meat to drool over (see how the other side of the "sexist" label works?) and two, they would have the same thought 99% of the rest of the Who audience would have: This is fucking stupid.
The Doctor is a man. Saying that he should be a woman simply on the personal idea that it would be "better" doesn't hold water to the facts of the show. If it's that important that you see breasts in front of those two hearts, write some Fanfiction. But, keep the politics and social trends out of Who...