4. The Rani, An Unscrupulous Scientist
The Rani, originally played with manic glee by Kate O'Mara, was a scientist who was exiled from Gallifrey due to being basically Gallifrey's answer to Joseph Mengle. She really didn't seem to be terribly interested in good or evil, or really care one way or the other about the Doctor or the Master (both of whom she met, but only one of which she kicked right in the TCE). Sadly, she also didn't care about morals or ethics. All she really cared about was science. It's easy to imagine the Time Lords bringing her back to Gallifrey and putting her to work making bioweapons to fight the Daleks. It's also easy to imagine what sort of genetic horrors she might have conjured up. That alone could make her an especially interesting character to revisit, especially if she was, say, trying to relocate one of those horrors to do something...unpleasant...with it.
Who Should Play Her:
Marina Sirtis. Yes, Counselor Troi off Star Trek: The Next Generation. She's a good actress, and she's got a nicely exotic look that could go well with the Rani. She's also proven that she can do a good job handling the vaguely scientific babbling that the Rani would be required to do. As an added bonus, she'd be a great lure into the series for Trek fans. And, frankly, given the right material, she's a pretty darn good actress.