Ace/Other realities

Timewyrm: Revelation
In an alternate timeline, Ace was killed by Chad Boyle in December 1978 when he hit her on the head with a brick. Boyle was subsequently recruited by the Timewyrm. This timeline was negated when the adult Ace prevented Boyle from attacking her younger self. (PROSE: Timewyrm: Revelation)

Matrix
In a timeline created by the Valeyard's use of the Dark Matrix while he was acting as Jack the Ripper, Ace became the Ripper's sixth victim when she and the Doctor went back to try and investigate the Ripper's crimes. This timeline was negated when the Doctor was able to defeat the Valeyard/Ripper and restore history. (PROSE: Matrix)

Klein's Story
In another alternate timeline, Ace was killed by Kurtz in Colditz Castle in October 1944. The laser technology contained in her Walkman was used by Nazi scientists to refine uranium and create nuclear weapons. They subsequently bombed New York City and Moscow, forcing the surrender of the United States and the Soviet Union and winning the war for Germany. This timeline was negated by an alternative version of the Eighth Doctor who, while posing as a German scientist named Johann Schmidt, fabricated a "flight log" for the TARDIS and manipulated Elizabeth Klein into travelling back in time to Colditz in October 1944 on the pretext of retrieving the Doctor so that he could teach her to pilot the TARDIS. Her lover Major Jonas Faber saw through Schmidt's trickery but was too late as Klein had already decided to disobey his orders and make the trip into the past. Unfortunately for Klein, her arrival in Colditz alerted the Seventh Doctor to the impending alteration of history and he and Ace were able to prevent it. (AUDIO: Colditz, Klein's Story)

Cyber-President's timeline
When the Cybermen allied with Rassilon to take over history, Ace was cyber-converted and confronted the Seventh Doctor while he was planting Nemesis mines across the Cyber-Fleet. (COMIC: Prologue: the Seventh Doctor)

The Domino Effect
In an alternate timeline where the computer was never invented, it was strongly implied that Ace became a young woman known as Dee, who worked with the amnesic Eighth Doctor to try and topple the corrupt Star Chamber that ruled Britain. (PROSE: The Domino Effect)

The Light at the End
In another alternate timeline created by, she met the Sixth Doctor and Peri Brown. She gave the Sixth Doctor the nickname of "Joseph," a reference to the musical Joseph and the Amazing Technicolor Dreamcoat. He did not understand the reference, though Peri found it amusing. The Sixth Doctor did not seem to be particularly pleased with his future self's choice of travelling companion. Ace got along well with Peri, who was surprised to learn that she referred to the Seventh Doctor as "Professor." Ace lost all memory of this encounter when the proper timeline was restored. (AUDIO: The Light at the End)

He Jests at Scars...
In a parallel universe, Ace would've been a companion of the Doctor, had the Valeyard not defeated him and merged with him in the Matrix. (AUDIO: He Jests at Scars...)