The Crystal Empire will form a Communist Party.
- Considering their history of being slaves themselves, once they discover that Equestria at large (minus Cloudsdale and other Pegasi cities) is OK with slavery, they'll go ballistic. Drawing IRL parallel, the USSR got started when the Russian people got sick of Czar Alexander II's bullshit and shot him, his family, and his dog, and formed an egalitarian society where all were equal. It didn't work in practice, but it was a good idea on paper; and if anyone can pull off institutionalized "share-and-share-alike," it'll be ponies. I forsee Cadence and Shining Armor being sent home, and Crystal pony street-preachers screaming about "each according to his ability, each according to his needs" in many a town square.
- And, that most beloved of Soviet traditions, invading.
- Alternatively, Cadence and Shining Armor will split off from Equestria and outlaw slavery in the Crystal Empire.
Humans aren't the only species the ponies have subjugated.
- Enslaving an entire species would go a long way towards legitimizing Fantastic Racism in general, so who's to say that Equestria hasn't gone after other targets? Though not enslaved outright, races such as Griffons may have been conquered and made into puppet states or vassals in the fic's universe.
- There's mention of a war with the Changelings at some point, though their ultimate fate remains unknown so far.
Given that all mention of them has been in the PAST tense, though... - I'd have to say griffons are quite safe from pony aggression since they have the Pegasus agility and a predator's vicious streak. They'd probably be able to tear off their master's face and show it to them before they could be choked into submission via slave collar.
- There's mention of a war with the Changelings at some point, though their ultimate fate remains unknown so far.
The events of A Dog and Pony Show will cure Rarity of her desire for a slave of her own
- Not only that, no legal ramifications will come of it since it's technically "Unclean Hands" (Forehooves? Forepaws?).
Appaloosa is in the vicinity of a large colony of escaped slaves.
- ...who viciously defend their borders.
A Sparatcus-esque rebellion with disgruntled slaves and sympathetic ponies will come into play.
Celestia hasn't shown much of a backbone over the course of the story, Luna isn't happy at all with that and the whole enslavement thing, and at the end of Chapter 15, Luna receives a note asking for her assistance against her "tyrant" sister. Plus the end of the Nightmare Moon arc reveals a colony of escaped/free humans living in the Everfree Forest. It would only be a matter of time before things tip over... The Diamond Dogs will be shown to have human slaves of their own
- However, the Dogs in the show proper seem to be more stupid than outright malicious; so "their" humans will be far more forgiving towards them. Said humans will, however, be unremittingly hostile; and perhaps downright cruel towards Rarity out of vengeance.
- Alternatively, the Diamond Dogs are Hiding a large number of humans from ponies. After all, Dogs are Man's Best Friend....
- Both can happen; several IRL human traffickers try to get their "product" into first-world countries so they can have a better life, as opposed to the more popular version of the profession (border-hoppers getting people over the Rio Grande from Mexico to America, for example); and all it takes is a rusty link in the chain for one to give people to the other.
Fancy Pants is running an Underground Railroad
Celestia is playing the Long Game
- She can't just turn around and outlaw slavery, but Twilight can. Since Celestia's grooming her as a successor, Twilight's first act will be to chuck slavery out on it's ear.
Discord will cause/ensure the success of a slave revolt
- Not out of the Goodness of his Heart, mind you, but there's nothing more chaotic than a revolution.
The human's ancestral homeland is the Wasteland from Fallout
Humans are immune to mind control.
- Considering that ponies brainwash problem creatures at the drop of a hat (Parasprites, vampire fruitbats, and each other when it suits them; all with the only thought put into it being "This is annoying me. I have magic and you don't, so you get to eat dark sorcery, space cowboy.") why don't they do so to their slave race? Why did it take 300 years of research by the nightmare moon cult to acheive "a breakthrough"?
Celestia will be called out... by the Council
She funds rebels against a decree she has no public stance against, had historically been the one to suggest it, and would have had to help enact it in the first place. Instead of dealing with the situation in the most expedient manner possible, she plots and waits over the centuries, leaving countless to suffer under inhumane laws, and aims to have the head of the Council assassinated, all because she doesn't want some unwarranted adulation. The Council will not be the good guys, but given that she plans to dispose of them anyways, they will at least call her out for her lack of spine. They might be a bunch of bastards, but that's the way they've been raised and the way things have always been for them, and they will at least stick by their decisions. Celestia instead is taking a ridiculously circuitous plan to avoid the spotlight despite the fact that all her claims of not wanting to be seen a tyrant makes her the opposite, a worthless, spineless coward unwilling to take charge despite the power she wields.