Merlin

Merlin may or may not have been/will become a future incarnation of the Doctor. It is suggested when (during his seventh incarnation) that Merlin Morgaine's version of the Doctor in her home universe. (DW: Battlefield)

A 'classic' interpretation of Merlin (as the elderly sorcerer) has appeared to the Doctor (in his fifth incarnation) at least once. (DWM: The Tides of Time)

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)

There is also some suggestion by both the Doctor and Muldwych that Muldwych may be the 'Merlin incarnation'. (NA: Birthright, So Vile a Sin, Happy Endings)