Merlin

Merlin may or may not have been/will become a future incarnation of the Doctor in an alternate universe. (DW: Battlefield)

Other information

 * Whilst stuck in Avalon the Doctor (in his eighth incarnation) attempted to pass himself off as Merlin to Queen Mab. (EDA: The Shadows of Avalon)
 * The Doctor had earlier met another, possibly different version of Merlin. (DWM: The Neutron Knights, The Tides of Time)