If they go and pick a black Doctor to seem equal-opportunities, the next Doctor after that will be a woman (the '24' school of equal opportunities).
It would be farcical. It would be a hundred times more racist to shoehorn in a ridiculous story about the Doctor turning into a black man (which in the Doctor's universe would have to be explained, on screen, in some excruciatingly PC way).
At least in 24 it served a purpose to the narrative, not just a novelty.
Personally I think the Doctor should continue to be played by relative unknowns. Established actors are likely to disappoint, especially if people just think that other character's they have played are somehow 'like' the Doctor (see Hugh Laurie, Anthony Head, etc.). David Tennant certainly didn't seem like a 'Doctor' type, and neither did Eccleston before him.