+1 for First Female Doctor
I think this would make for an interesting development, and there are a few strong contenders depending on what sort of personality the 12th Doctor is to assume.
As already mentioned, Sue Perkins would make a good 'quirky' option. Helen Mirren would undoubtedly be fantastic under any circumstances, although I'm kind of torn between picturing her roles in Red and Arthur. Emma Thompson would also potentially be a good choice as a no-nonsense, matronly Doctor.
However, while all of the above would definitely be fine choices, part of me would quite like to see Gemma Arterton as The Doctor... preferably in the style of her role as Kelly Jones in St Trinians. A nice opportunity for some initial post-regeneration confusion about her memories and feelings towards Clara, maybe leading to a kiss... guess it's unlikely, but certainly something I can dream about! ;)