5. Helen Mirren
When conceiving the various specials that would serve as David Tennants swan song as the Doctor, Russell T. Davies wanted one of the companions to be a strong older woman. Helen Mirren was high on his list for a potential guest appearance. He sketched a character outline of a Russian space station commander but later changed the nationality to English because Mirren had played a Russian in the film 2010. Lindsay Duncan wound up playing the role. Had Mirren been cast, she would have been the first Dame to appear in Doctor Who. And awhile back, it was rumored that she wanted to be the first female Doctor.