The Doctor's final incarnation

By some accounts, the Doctor eventually reached their final incarnation and then subsequently met their death.

Knowledge of the final Doctor
When Ruath used a Time Scoop to view the Doctor's time stream, she advanced the viewer to the Doctor's final face. After remarking "So that's what you become! Well, that's no good to me," she shifted the Time Scoop to the Fifth Doctor, whom she thought would be easier to manipulate. (PROSE: Goth Opera)

The Tremas Master told the Sixth Doctor that the Valeyard was "an amalgamation of the darker sides of your nature, somewhere between your twelfth and final incarnation". (TV: The Ultimate Foe) This phrasing would be repeated by the Seventh Doctor (PROSE: Matrix) and the historical text A Brief History of Time Lords. (PROSE: A Brief History of Time Lords)

The Doctor's thirteenth incarnation
By various accounts, the Doctor's final incarnation was the thirteenth of their original regeneration cycle.

By one account, the Tremas Master said that the Valeyard came from between the Doctor's twelfth and thirteenth incarnations. (PROSE: The Ultimate Foe)

In a story related by the Valeyard, the thirteenth and final incarnation of the Doctor supposedly composed a set of scrolls which detailed his work to bypass the limit of twelve regenerations and that the Valeyard himself was created as a Time Tot on a mud planet orbiting Etarho as a result of the Doctor's experiments. The Valeyard's claim, however, was doubted by the Sixth Doctor. Indeed, the Doctor found what appeared to be his final incarnation on Etarho to have been the Valeyard masquerading as him. (AUDIO: Trial of the Valeyard)

When Ace entered the Doctor's mind, she saw that it had space for thirteen total selves, with the incarnations beyond the Seventh Doctor still being Watchers yet to be molded into distinct incarnations, including the final thirteenth incarnation. (PROSE: Timewyrm: Revelation)

The Relic
By one account, the Doctor's final incarnation appeared to have died in Dronid, and became The Relic, to enter into an auction on Earth. (PROSE: ''Alien Bodies)

Ginger Doctor
According to one account, a ginger incarnation of the Doctor (PROSE: ) believed himself to be the last. Daughter of Mine mentioned him to the Thirteenth Doctor, who did not recognise this incarnation. (AUDIO: Shadow of a Doubt)

Behind the scenes

 * Paul Cornell, who mentioned the Doctor's final incarnation in several of his stories, wrote a depiction of the Doctor's final life in his short story The Last Doctor.
 * Craig Hinton's Doctor Who lore indicated that the Doctor's thirteenth and final incarnation would survive after death as the Red Guardian.
 * The unproduced comic story The Last Regeneration would have featured the final incarnation of the Doctor being ginger.