I don't think it's going to be anyone we've ever heard of. Doctor Who has traditionally been the show that makes people careers.
Also, I don't think it's going to a women, as cool as that would be. Ginger, maybe, but not a women. I think it would be too difficult for the show to explain how his regeneration affected his/her gender. There are female timelords, he had a wife, a daughter, and a mother. These people are never explained to be bi-gendered, always being refereed to as she or her.