9. He Will Knock Four Times
When The Master appeared in season three, it seemed he was defeated by the end, but we knew he'd be back after the mysterious woman retrieved his ring from the ashes of his pyre. She would, of course, go on to revive the Master for the 2009 special, The End of Time, which aired two-and-a-half years after the Master's last appearance.
During the course of the specials leading up to this story, The Doctor learns of the prophecy that claims He Will Knock Four Times. Fans instantly connected this to The Master. However, when the moment came, it was actually Wilf who knocked four times after being trapped in a glass case that is going to fill with radiation, leaving Ten in a moral predicament. He thought he had cheated death, and was suddenly confronted with the fact that he had to die to save his friend.
This twist of fate had been a long time coming. He met Donna in 2006 and came across Wilf in 2007. He found Donna again and travelled with her again in 2008, then dropped back into Wilf's life in 2009. Fate was pulling him towards this for a very long time. Even more surprising, is it looks like they're being pulled back together now, 14 years later.