The Doctor's reality (The Curse of Fatal Death)

In one version of the Doctor's universe, the ninth incarnation of the Doctor was engaged to be married to his companion Emma. (TV: The Curse of Fatal Death)

A day to come
The Eighth Doctor looked into a Tomorrow Window and asked to see his future. One of the possible ninth incarnations of the Doctor was a "listless-looking" man on a sofa beside a girl in a red dress in a medieval dungeon. (PROSE: The Tomorrow Windows) When examining the Doctor's timeline, Marnal exclaimed to Rachel that the Doctor had "three ninth incarnations". (PROSE: The Gallifrey Chronicles)

The Doctor's future
At some point, the Doctor began travelling with the "glamorous" Emma (PROSE: Who's After Your Cash) in whom, "without even knowing [he] was looking", he found "a woman to love". The two developed a romantic relationship, with the Doctor's next incarnation later indicating that the two had had sex at least once, identifying her as the only traveling companion he had ever "had". (TV: The Curse of Fatal Death)

Coming back to Earth for the first time in many years, the Doctor became aware of a charity event centred around a "TV spectacular" which, among other things, contained a depiction of his and Emma's adventures battling the Daleks and. He wrote a rambling letter to The Mirror endorsing the production and insisting that readers give what they could to those in need. In this letter, he described himself as "over 800 years old". (PROSE: Who's After Your Cash)

Behind the scenes

 * In 2021, when discussing how elements of The Curse of Fatal Death had made their way into Doctor Who proper, Steven Moffat remarked that he should have asked Chris Chibnall to include footage from the special as the natural next step to having "canonised" the Brain of Morbius Doctors.