Enough with the Chiwitel Ejifor/Idris Elba stuff. I wouldn't cast a black actor, simply because of that bloody academic paper.
And a female Doctor would be weird. Sorry of I seem sexist and racist, but no. You wouln't get either of those guys or Tilda Swinton, they'd want too much money.
That said... Lenny Henry might be interesting. And Lara Pulver (Irene Adler, 'Sherlock') would be good if she could bring that same energy to the show.
Was chatting with a chap at my bookclub meeting yesterday. We bemoaned the downturn in the quality of recent Who, and he had an interesting take: that they are trying way too hard to appeal to the US market, to rake in even more dollars. The show has definitely changed, there's something missing. And if this is correct, they won't change the Doctor from being a white male, not wanting to rock the boat.