In “Mirror, Mirror,” Good Kirk tries to persuade Bearded Spock that there is a different way to run a space-ship, and an inter-planetary empire. Good Kirk also lets him in on the secret of Bad Kirk’s Tantalus Field. When Good Kirk and company are transporting away, Bearded Spock is left considering it all.
So what happens next, when Bad Kirk and his gang transport back aboard the ISS Enterprise?
Does Bearded Spock just settle back to his previous routine, content to be the Chief Science Officer, and not challenge Bad Kirk?
Or does he try to take out Bad Kirk? Does he succeed?
And if he does succeed, can he go further and bring down the Terran Empire and lead the remnants to a Federation-type system?
In Deep Space Nine, they returned to the 24th century Mirror Universe once a season from season 2 onward. It was explained that Spock became the leader of the Terran Empire and made it less warlike. But that allowed a Klingon-Cardassian Alliance to rise up and conquer Earth and subjegate humans (and Vulcans). The humans formed a rebellion with occasional help from the main universe characters.
And, no doubt, when the Klingons and Cardassians were defeated, a new Terran empire would have formed - probably even more xenophobic than the original, as a Vulcan had set things up so that they were subjugated by Klingons and Cardassians (and Bajorans).
Some of the Mirror Universe novels are a hoot. There is a two-volume set called “Dark Passions” or something like that which focuses on all the women characters. Very sleazy, and I mean that in a good way!
“Dark Mirror”, by Diane Duane, is excellent. It’s a TNG spinoff novel, and doesn’t match up with the Mirror Universe of Deep Space Nine, though, in that it establishes that the Empire is still strong. I think that from what Picard was able to figure out, Mirror Spock did take over the Enterprise and become a power player in the Empire starfleet - wasn’t entirely clear from the historical records if he was really planning to make things better or was just using the Tantalus field for personal gain like Kirk had. Then Spock mysteriously disappeared - no explanation, but presumably somebody else managed to outplay him and avoid getting Tantalized.
William Shatner makes use of the Mirror universe in his series of novels in which Kirk is resurrected after his death in Generations and becomes friends with Picard. In them, his counterpart from the Mirror universe has become the Emperor Tiberius and attempts to take over Kirk’s universe, as well.