A future incarnation of the Doctor existed at some point beyond the Doctor's eighth incarnation.
The Doctor visited the City of Glass with Iris Wildthyme and a "black boy in a silver, figure-hugging spacesuit". In the City of Glass, the Doctor fought giant owls with an elephant gun. Records shown by Marn to the Eighth Doctor many years later showed that on this visit, this Doctor spoke the words, "TARDIS", "Time Lords" and "Gallifrey". (PROSE: The Blue Angel)