We almost got a black Doctor, as Patterson Joseph was one of the front runners, before Matt Smith showed up.
I love the actor so would have been thrilled it get him as a Doctor ( Go watch 'Neverwhere' to see what is basically his audition, in my mind) and then you get the bonus of people's heads exploding at the idea of a black Doctor.
Not a huge fan of the lady Doctor idea, but with a great actress and a solid writer, I could be convinced.
My idea is that after regeneration #12, in order to avoid going all crusty and 'Deadly Assassian Master looking, the Doctor's body goes through a radical process where he changes genders.
Or it's something he set up to avoid becoming the valeyard.
You could even say it radically alters his mind as well, so you'd have the mystery sub-plot of the Doctor not being sure what's happened.
There you go, Mr. Moffatt, free of charge. Just give me a mention in the closing credits.