3. The Source of All Evil - Charmed
The Doctor versus the powerful demon ruler of the underworld - what could be more cool? Magic is a strange subject in the Whoniverse. It's a concept that was once considered to be a fundamental force of the universe, which was then almost entirely replaced by science and rumoured to no longer exist. As recently as the tenure of the tenth Doctor (David Tennant), spells, rituals and incantations were among things he didn't believe in at all, so it would come as something of a surprise for him to realise that magic existed and was as powerful as ever. An interesting storyline, therefore, would be for the powerful magic user - The Source of All Evil - to rise from the underworld to provide an unsuspecting Doctor with a foe more powerful, more ruthless and more unique than anything he has ever encountered before. The list of powers possessed by the Source is huge and includes (but is by no means limited to) force fields, invisibility, energy blasts, mind manipulation, dimensional traversing, super strength, super speed, telepathy, technopathy and telekinesis. The good Doctor would have his work cut out, for sure.