I've been re-watching the seminal Doctor Who adventure The War Games, an episode a night, for the last week. People say it's too long and dragged out, but I've always liked it a lot. And a main reason for that is the evil Time Lord, the War Chief. Quite brilliantly played by Edward Brayshaw, very much like Roger Delgado at his best, he's confident, intelligent, nasty, a great villain all round.
I do like the theory that he's an earlier incarnation of the Master, fannish though the idea is, and even though it clearly wasn't intended by the writers at the time (the War Chief is shot dead towards the end of his only adventure, and it was many years before it was decided that a violent death causes a Time Lord to regenerate into a different actor, rather than just to die) - but really, what's the point of being a Doctor Who fan if you can't make up your own back story for a favourite character?