5. The First Doctor
William Hartnell's original Doctor was a mysterious, ruthless and patronising character, often described nowadays as a cantankerous old man. Reminiscent of anyone? He's arguably the closest match to Capaldi's current Doctor as they both have that grumpy old man streak running through them. To see both of them working together in an episode would be a joy to witness. Well, 'joy' might be the wrong word, because what happens when you put two cantankerous old men together in a room? Explosions? Hilarity? Basically it would a real grumpy time for all concerned. Obviously, with the fact that unfortunately Hartnell passed away a long while ago back in 1975, this would never happen - although David Bradley did put in a stellar performance as him in An Adventure in Space and Time, the TV film depicting Doctor Who's origins. His performance won a lot of praise from fans with some even showing enthusiasm for Bradley to depict the First Doctor in Doctor Who itself. Will it ever happen? Well, no. But it certainly would be great if it did.