The Doctor's ninth incarnation

By most accounts, the Doctor's ninth incarnation was the one who followed the Eighth Doctor's regeneration.

When examining the Doctor's timeline, Marnal exclaimed to Rachel that the Doctor had "three ninth incarnations". (PROSE: The Gallifrey Chronicles) The Eighth Doctor's visions of his future included as many as ten potential ninth bodies, (PROSE: The Tomorrow Windows) and accounts differed as to which one he eventually regenerated into. (TV: The Name of the Doctor, PROSE: Doctor Who and the Time War, et al.)

In addition, some accounts of the Doctor's early life suggested that the so-called First Doctor, from whom the numbering allotted to the conventional Eighth Doctor started, (PROSE: The Eight Doctors) was not actually the Doctor's first incarnation. (PROSE: The Power of the Daleks, Doctor Who and the Brain of Morbius, TV: The Timeless Children, et al.)

Listless-looking man


One of the faces the Eighth Doctor saw in the Tomorrow Window was "a listless-looking man." (PROSE: The Tomorrow Windows) By one account, the Doctor's ninth incarnation had a companion named Emma, (PROSE: Who's After Your Cash, TV: The Curse of Fatal Death) with whom he fell in love; they decided to stop adventuring and settle down together. The Doctor met the Master on Tersurus to inform him, but the Master brought the Daleks. This Doctor ultimately regenerated into tenth, eleventh, twelfth, and thirteenth incarnations after being shot with Dalek energy beams. (TV: The Curse of Fatal Death)

Man with big ears


After the Eighth Doctor saw several visions of his future in the Tomorrow Window, a single figure solidified out of the blur: a gaunt, hawklike face that gave him a broad, welcoming grin. (PROSE: The Tomorrow Windows) According to some accounts, the Eighth Doctor regenerated into this incarnation after using the Moment to end the Last Great Time War. (COMIC: The Forgotten, PROSE: Doctor Who and the Time War) He travelled with Rose Tyler and Jack Harkness. (PROSE: Dr. Ninth, et al.)

Warrior


By many accounts, the Eighth Doctor regenerated into a ninth incarnation who renounced the title "Doctor" while fighting in the Last Great Time War. (TV: The Night of the Doctor, The Day of the Doctor, PROSE: The Day of the Doctor, et al.) Later incarnations disavowed this version of themselves for breaking the promise of their shared name. Their last incarnation of their first regeneration cycle labelled himself the "Eleventh", ignoring his war incarnation; (TV: The Name of the Doctor) however, this Doctor's final act was to actually save Gallifrey with his other twelve incarnations (TV: The Day of the Doctor) before regenerating into the aforementioned incarnation which travelled with Rose Tyler. (PROSE: The Day of the Doctor)

Others
Other possibilities included a figure in a velvet suit and eyeliner, a man with curly hair and a lopsided smile, and a pale aristocrat. (PROSE: The Tomorrow Windows)

Other possible successors to the Eighth Doctor were incarnations whom the Doctor already knew lay in his future: a ginger incarnation (PROSE: The Tomorrow Windows) whose adventures intersected the Seventh Doctor's several times, (PROSE: Battlefield, Transit, Birthright, Happy Endings) and the Valeyard, (PROSE: The Tomorrow Windows) who repeatedly menaced the Sixth Doctor (TV: The Mysterious Planet, AUDIO: The Brink of Death, et al.) and was described as an amalgamation of the Doctor's darker sides from between his twelfth and final incarnations. (TV: The Ultimate Foe)

One possibility was that the Eighth Doctor's successor was not a proper incarnation of the Doctor but another of the four surviving elementals, the man with a bent nose. (PROSE: The Tomorrow Windows, The Gallifrey Chronicles)

Another possibility was that the Eighth Doctor's future included many men in pseudo-Edwardian dress as well as the First Doctor. (PROSE: The Tomorrow Windows) One incarnation of the Doctor resembled the Eighth Doctor with short hair but lived on Gallifrey and had forgotten many of his adventures. (PROSE: The Infinity Doctors) By one account, the old man and young girl who landed their spaceship in the junkyard at 76 Totter's Lane were Soul and Zezanne in the shape-shifting Jonah; in their amnesiac states, Zezanne believed that Soul was her grandfather. (PROSE: Sometime Never..., TV: An Unearthly Child)

Behind the scenes

 * The reference to the Doctor having "three ninth incarnations" in The Gallifrey Chronicles is a reference to Rowan Atkinson's Ninth Doctor in The Curse of Fatal Death, Richard E Grant's Ninth Doctor in Scream of the Shalka, and Christopher Eccleston's Ninth Doctor in Series 1.
 * Each of these incarnations had been previously alluded to in The Tomorrow Windows.