Let's try a little something more with these.
For thousands of years, Princess Celestia has ruled Equestria with kindness, wisdom, and patience. No matter what threat approaches her country or what manner of natural or political disaster may strike she's always handled herself with grace and never complained once when protecting and guiding her little ponies. The job hasn't been easy, not by a long shot, but she's always been proud of her work.
Of course, that doesn't mean she can't desire a chance to stretch her wings. Something Discord is all-too-aware of.
Oh it sounded like one of Discord's jokes at the start. Just him trying to get a cheap laugh by suggesting she take a vacation for a while, then he'd teleport her to Las Pegasus or Manehatten and she'd need to balance an impromptu public appearance while simultaneously trying to find an opening to return to Canterlot as soon as possible.
Except Discord might have taken the joke a little too far this time. Our beloved Princess of the Sun certainly wasn't expecting him to send her to an entirely different universe, and she especially didn't expect him to stick her in the body of her counterpart in that world! Now she's just Celestia, a pegasus pony and older sister to the freshly crowned Princess of Friendship, Luna.
And as it turns out, plain-old regular Celestia might not have been the nicest pony in this world...
Heh, the classics of the Mirror-verse can always be fun to play with. Rather than just borrowing Mirror-questria from the comics, though, this would be one of my own design. Swapping not just alignments, but also stuff like species, life position, and relative power levels between the heroes and villains of the canon, both major and minor ones.
So you get Luna (remember, she used to be Nightmare Moon), Discord, Tirek, Chrysalis, Sombra, and one of the Dazzlings as the normal Equestrian citizens who found the Elements of Harmony.
The Mane Six and Spike each wind up taking the place of one of the Season Finale big bads (such as, say Empress Rarity, wicked mistress of the Crystal Empire).
The minor villains (Trixie, Flim and Flam, et al) wind up in more prominent heroic positions such as being the ruler(s) of this Equestria.
And major forces for good (like Celestia herself) are the minor villains. In this case, this Swap-verse Celestia is an obnoxious show-offy pegasus pony that's jealous of her little sister Luna being a big hero and stealing attention from her.
So you get Celestia trying to figure out how to get home when she's considered to be an untrustworthy jerk (as a best-case scenario), coupled with the dawning realization that if she's in Swap-lestia's body in Swap-verse then there's only one pony that could be occupying her original body back in Canon Equestria...