In one version of reality, the Seventh Doctor became the "last of the friendly Time Lords" and ultimately seemingly sacrificed himself in a final stand near Stonehenge to defeat a rogue Time Lord known as Tannis. In his absence, Ace, who had undergone training with the Doctor's old friend Casmus, was officially made the new Time Lord by the Kingmaker while the Minister, despite having lost his status as a Time Lord, continued to travel in the universe using the formula for doors.
"I am the course of Time. I am a God of the Fourth. Now die."
"But… but you'll die too! You'll die too!"
"I've been dead before." (WC: Death Comes to Time)
When exposed to anti-time, the Eighth Doctor of the positive-time universe saw a reality where the Time Lords had "terrible mind powers", (AUDIO: Zagreus) matching the mysterious powers which, in this account, Time Lords were said to inherently possess as "Gods of the Fourth" despite being sworn never to use them. (WC: Death Comes to Time) According to one account, in the post-War universe of the Eighth Doctor, Tannis was a threat Gallifrey had yet to face, seemingly establishing the events involving the Seventh Doctor as a different timeline from his, (PROSE: The Gallifrey Chronicles) though another account dealing with this same version of the Eighth Doctor acknowledged the Canisian invasion. (PROSE: Trading Futures) As the Seventh Doctor invoked his power as a "God of the Fourth" to destroy himself and Tannis, Tannis desperately cried out that the Doctor's actions would "disrupt the course of time", which the Doctor did not deny. (WC: Death Comes to Time)
Behind the scenes
The place of Death Comes to Time in wider Doctor Who continuity was a subject of controversy was a point of contention from the moment it was first release, due to it featuring an apparent death of the Seventh Doctor incompatible with his death in San Francisco and regeneration into Paul McGann's Eighth Doctor in the 1996 Doctor Who TV movie.
This drove the Doctor Who website's official section for the then-recent webcast to include an entire page devoted to "explaining" the perceived contradictions between it and wider Doctor Who continuity, although there was a disclaimer that this was an "unauthorised guide to Death Comes to Time" whose "theories" were purely the personal contentions of the article-writer, and may turn out to be "inaccurate, wrong or silly". The article pointed out that nothing in the webcast actually contradicted any element of televised lore up to Season 26 of the TV series, despite the perceived radicalism of the depiction of the Time Lords as having secretly possessed "magical mind powers" they had never used before. Indeed, it could be read as consistent with the entire Virgin New Adventures line, so long as one assumed that the mission at the end of Lungbarrow was not actually the direct lead-in to the TV movie that it was written as, and that the Doctor instead wound up travelling with Ace again, joined by Antimony.
However, as regarded the death of the Seventh Docto, the article theorised that Death Comes to Time was actually an aborted timeline: in using his Time Lord powers to slay Tannis, the Doctor "disrupted the course of time" (as Tannis himself said in dialogue), creating a paradox by averting his already-fixed future death in San Francisco. This paradox is what destroys Tannis and the Doctor both. The article further speculated that Ace's duties as a newly-minted Time Lord would involve reversing the damage this had done to the timeline, making the entire Death Comes to Time timeline self-resolving and linking back to conventional Doctor Who continuity.
Sylvester McCoy regenerat[ing] into Paul McGann in the TV Movie [was] the established order of things. But I think we can tell from Tannis's dialogue that the Doctor is altering his own future by destroying himself on Salisbury Plain ("But you'll disrupt the course of time... you'll die too."). And it's the paradox that he creates that he destroys himself with. Or, at the very least, he takes himself out of time. (…) The TV Movie did happen. Paul McGann was the next regeneration of the Doctor. But time has now been changed by the Seventh Doctor on Salisbury plain. I would guess that putting things right is part of Ace's quest.