Warning: SPOILERS for Star Trek: Discovery Season 3, Episode 10 - "Terra Firma, Part 2"
Emperor Philippa Georgiou's (Michelle Yeoh) return to the Mirror Universe in Star Trek: Discovery season 3 had a curious side effect that essentially turned the Terran ruler into what the Mirror Spock (Leonard Nimoy) became in Star Trek: The Original Series. "Terra Firma, Parts 1 and 2" was Georgiou's swansong from Star Trek: Discovery as the Emperor was written off in grand style; Carl (Paul Guilfoyle), who revealed himself to be the Guardian of Forever, sent Philippa to an undisclosed point in the past when the Prime and Mirror Universes were still aligned, and Commander Michael Burnham (Sonequa Martin-Green) believes Discovery will "never" see Georgiou again.
Along with the Guardian of Forever, a guest star from TOS, Georgiou's behavior when she spent three months back in the Mirror Universe echoed what is known about the Mirror Spock after his lone appearance in TOS' "Mirror, Mirror". In the original Mirror Universe episode, a transporter accident switched Captain James T. Kirk (William Shatner) and three of his officers with their Mirror counterparts. Aboard the Terran Empire's I.S.S. Enterprise, Kirk had to pretend he was his evil doppelganger as he tried to figure out how to return to the Prime Universe. But crucially, after learned about the doomed Terran Empire from the Mirror Spock, Kirk urged the goateed Vulcan to act to save the Mirror Universe's future before the Captain returned to his proper reality.
When Star Trek: Deep Space Nine picked up the Mirror Universe's story a century afterward, fans learned that Spock indeed took over the Terran Empire and tried to steer it from fascism into a more peaceful state. Unfortunately, the Vulcan's changes proved disastrous; the Terran Empire had made too many enemies and the Klingon/Cardassian alliance, aided by other worlds like Bajor, conquered and enslaved the Terran Empire. When DS9's personnel began crossing over into the Mirror Universe, they found the Terrans and other subjugated races like the Vulcans fighting a desperate resistance against the Alliance, which was led by the Regent, who was revealed to be Worf (Michael Dorn). By Star Trek: Discovery's 32nd century, the fate of the Mirror Universe is unknown because the parallel reality had broken away from its connection to the Prime universe and no crossovers had occurred in 500 years.
When Emperor Georgiou returned to the Mirror Universe in "Terra Firma", she was no longer inclined to be the despot she once was, thanks to the years she spent living in the Prime universe's United Federation of Planets - although according to the Mirror Michael Burnham, Georgiou was already growing "weak". Regardless, because Georgiou had changed and had "seen the future", she began to implement changes to the Terran Empire to suit her more "glorious", more Federation-like vision. Georgiou was still not above manipulating Michael to assassinate the conspirators planning a coup against the Emperor, but Philippa also showed mercy to and befriended her Kelpien slave out of respect to his Prime universe doppelganger, Captain Saru (Doug Jones).
But Georgiou's vision of a better Terran Empire failed; Burnham betrayed her yet again and they ended up killing each other before Philippa "passed through" the Guardian of Forever's portal and returned to 3189. Had she been able to see them through, the changes Georgiou planned for the Terran Empire would likely have been as equally disastrous as Spock's. Not only did the Terran Empire have too many rivals out for their blood, but Georgiou also may not have been able to override the vicious nature of her fellow Terrans in her bid for a different future.
There is also the greater question of what actually happened in "Terra Firma's" Mirror Universe and whether Georgiou altered the past in her three months back in her original reality. Many people died earlier than they did in Star Trek: Discovery season 1's Mirror Universe four-parter, which takes place months after "Terra Firma" - including Georgiou herself, apparently. Did Georgiou actually change the Mirror Universe's past, which would then affect the trajectory of everything going forward, or did she create a branching alternate reality within the Mirror Universe akin to what the Kelvin timeline is to the Prime Universe? Either way, the Mirror Universe is apparently just not built for peace, and Emperor Georgiou's attempts to create a brighter future had a catastrophic result, just like when Mirror Spock tried it.