Break the mould?

A female Doctor Who?  Why not??  It's not like James Bond, where you are to a certain extent constrained by the books in which Fleming describes his main character in some detail.  There are no books on which Doctor Who is based and it is a fundamental characteristic of the Doctor that he regenerates from time to time so why not as a woman?  Indeed, let's go the whole hog and have him regenerate as a black woman! How about Naomie Harris?