Merlin

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

Resolution
The 'Merlin Paradox' was apparently resolved when the Eighth Doctor travelled to Arthur's reality, aiding in his birth; as the Doctor discovered, he was only the second Merlin, the other one being a renegade Time Lord who gave Arthur and his kingdom advanced technology, prompting Morgaine to seek an allaince with them. The other Merlin, disguised as the Doctor, was imprisoned in the Arctic by Morgaine after she was temporarily banished to a desert by the Doctor, leaving the Doctor to set up the events that would later be experienced by his seventh incarnation. (ST: One Fateful Knight)

Other information

 * Whilst stuck in Avalon, the Doctor 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)
 * It has been implied that Muldwych is the future incarnation of the Doctor who is Merlin.