What I'm thinking is, in episode 23, during Twilight's flashback, she goes all glowy-eyed and it looks like she might not snap out of it until Celestia intervenes.
So what if she didn't? A perverted Element of Magic could be a Reality Warper with Power Incontinence, exactly like what Twilight almost was.
Yeah, see, this is how I would tie everything together in the "mirror universe": Have all the personality shifts stem from Princess Celestia actually being a cruel tyrant. And it's really easy to come up with twisted versions of the main 6 because this show's not afraid to let them all hit rock bottom a couple of times, meaning you just have to imagine they have mental breakdowns and reach their lowest points but then
stay there, as no one cultivated a friendship to pull them out of it. So maybe Twilight Sparkle never got into Evil Celestia's good graces, and now Twilight's a paranoid recluse who abuses her magic powers to get others to do what she wants, whose plans never make sense, and who comes up with wild ideas about her enemies coming to get her. (Maybe the library isn't really hers to live in in this timeline, but she does anyway because who can stop her?!) Pinkie Pie would be the other crazy recluse, perhaps sitting around in a messy, forgotten room of the Sugarcube Corner all day, with no friends at all because she can't help scaring everyone she meets with her disheveled appearance and random, despairing laughter.
Then you could have Fluttershy and Rainbow Dash as the ruthless enforcers of the regime, with Rainbow Dash getting a thrill out of hunting down suspected rebels and doling out painful amounts of lightning, and Fluttershy only using her "motherly" shtick as a pretense to abuse the young and the small for not living up to some spartan standards. You could have a scene where she shames the Cutie Mark Crusaders for not having earned their marks yet!
Applejack and Rarity would then be the status-seekers who no longer enjoy working hard and use their connections to reach positions where they can have everything done for them. Because farm work is hard, especially now that the princess is extending daylight hours yet again, so Applejack might as well be the one working the workers, preferably from a cushy office! It would be great, too, if this Rarity had married Prince Blueblood and now devotes her time to buying more and more pretty things to make herself look good, totally shunning the common pony.
The really great thing is that the "bad guys" might have turned out more humble because of this too. I once had this idea that Trixie's history was similar to Rainbow Dash's: She went to a magic school for a while, but got kicked out for her showboating and bullying. So in the mirror world, Trixie could have wanted to put on a good, clean show, but flunked out of magic school because no one saw how that kind of magic would benefit the defense of Equestria. Whereas Twilight Sparkle is now the bully, but she's also the real deal in terms of powerful magic, and she sees herself as being in competition with Princess Celestia. I think that would help viewers appreciate what a difference a kind and just leader makes.