I always see the Doctors as non-sexual anyway (ie, never noted any sexual chemistry between any doctor and any companion, never shipped a single pairing!) And that makes sense to me because of the Doctor's great age - sex has been outgrown.

My great dream for a Doctor would be Tilda Swinton - it is absolutely believable to me that she could form a link in the current Eccleston-Tennant-Smith-Capaldi continuum. Would love to see her with a young male companion as well.

I think the more traditional side of fandom would have a good shriek of outrage and then get used to it. After all Swinton has already regenerated from man to woman (Orlando!)