5. Romana
With all the bickering going on lately about whether or not the time is right for a female Doctor, I would argue that we've already had one. Her name is Romana, and she was glorious. Last time we saw her, she had decided to stay behind in E-Space, to fight against injustice and get away from Tom Baker because they had a messy divorce. We don't really know too much about E-Space, except that it uses negative coordinates and it's nearly impossible to get to from our universe. But it'd be fun to watch her tool around this odd little universe with K9 in tow, exploring and getting into trouble just like old times. After all, she didn't seem to be in too much of a hurry to head back to Gallifrey, so maybe this is her chance to cut loose a bit. Lalla Ward was often at her best when she was given the chance to carry an episode, playing the Doctor rather than the companion role. I would have loved to see her on her own, out from under the Doctor's wing at last.