3. Romana, One Of The Doctor's Companions And Possibly Lord President
Romana was a Time Lady who traveled with the Fourth Doctor. Initially played by Mary Tamm, she helped the Doctor to look for the Key to Time. Later, she regenerated for reasons never really explained, and in this new form was played by Lalla Ward, who later went on to marry Richard Dawkins. Yes, really. Romana was a great character for knocking the Doctor down a peg or two, as do many of the other great companions. She was young by Time Lord standards, and made no bones about the fact that she'd excelled at school, scoring better grades than the Doctor had, much to his chagrin. She was a great foil. In the audio stories, and other spinoffs, she went on to become Lord President of Gallifrey, even getting her own audio series with Leela and not one, but two K-9s. She also may have been Lord President during the start of the Time War.
Who Should Play Her?
There's a little-known British actress who deserves much more exposure. Her name is Lisa Bowerman. She's been in Doctor Who before. She played a cat creature (more or less...it's complicated), in the very last old series story, "Survival". She has since gone on to act extensively in the Big Finish audio range, playing a Doctor Who spinoff character named Bernice Summerfield. She has also appeared in many other TV shows. I think she'd be an excellent choice to play a charming, witty, and perhaps vaguely befuddled version of Romana. But this would be a charming, witty, vaguely befuddled version in the same way that the Second Doctor was those things. Underneath, there would be a core of hard steel, and a strength ready to be wielded at a moments notice.