Certainly a long shot, but why the hell not? Now that Sir Ian McKellen has completed his role as Gandalf in the Lord of the Rings/Hobbit trilogies (with the final one being released this Christmas), it's - somewhat more - possible that he could fit Doctor Who into his busy schedule. He managed it with Coronation Street, after all. For a series that prides itself on keeping itself fresh, two gruff old men travelling the universe is not a combination that's been seen on the show before. With his serious acting chops and deep gravitas, McKellen could certainly handle the role of the Time Lord. This is a list for companions, after all, so perhaps he could be one of the Time Lords rescued whenever the Twelfth Doctor eventually gets around to rescuing Gallifrey? He could be one of the Doctor's friends from his school days, for example. The series has left that section of the Doctor's life purposefully vague so it wouldn't be stretching the parameters of the show to fit McKellen into it.