Info: A while back I saw someone mention something about a Slytherin in book four named Cassius Warrington, who also put his name into the goblet of fire. Here's my interpretation on what might have happened if he'd been picked as champion instead of Cedric.
"And the Hogwarts champion is…" Dumbledore pauses for a moment, students of all houses waiting in tense silence for the announcement. His navy sleeves move slightly as he opens the small piece of parchment, and his face falls a little bit. Students clad in green and silver, red and gold, navy and bronze, and yellow and black all grow more and more intrigued. It's clearly not who he thought it would be, so who is it?
"The Hogwarts Champion is Cassius Warrington."
Gasps fill the hall, followed by a shocked silence. No one can believe who the champion is. It's not a bold Gryffindor, or a kind Hufflepuff, not even a wise Ravenclaw. Instead it's an 'evil' Slytherin. Another moment of silence. And then? Chaos.
"NO!" the word choruses through the great hall from three of the four houses, outraged cries and vicious demands for a retry. Because they can't have a Slytherin as their champion, they just can't. But Cassius joins the other champions with his head held high and doesn't let their harsh words get to him. He's a Slytherin after all, he's been dealing with this shit since he got sorted. He wonders vaguely how all the other champions are preparing so studiously in the libraries when they've not even been told what the task is yet. Well, until Harry Potter walks up to him one sunny afternoon.
"Dragons," the Boy who Lived whispers, as if not wanting to get caught with a Slytherin. Cassius understands now, that everyone else already knows it's dragons, have found out somehow. And he's grateful for the boy who went past house affiliation to give him a fair shot at the first task. He gladly begins preparing and comes out second, much to everyone's dismay. A month later he walks up to the Golden Trio, Hermione and Ron jumping up with their wands drawn. Ready to fight him, but he just looks at Harry and returns a favor.
"Try sticking the egg underwater," he mumbles, walking back off to his table. Ignoring the curious gazes of the table clad in navy blue, full of Ravenclaws with their navy striped robes, and students from Beauxbatons drenched in the deep navy color. He pays them no mind because Slytherins never forget those who've helped them out. It's only right that he would return the same courtesy Harry had extended to him. And then he's first place, and students merely glare at him. At least it's a Hogwarts student that's first, even if he's a Slytherin.
In the maze Warrington doesn't care who sees or who notices, this boy is only fourteen and he needs all the help he can get. So they help each other through the maze, getting to the cup and being equally as shocked when they're transported to a graveyard instead of out of the maze. Cassius knows something's wrong immediately, standing in front of Harry without a doubt. Because Slytherins are fiercely loyal, and they'll protect those who have protected them. Harry is devastated as Voldemort orders Peter to kill the spare. He's horrified. Voldemort doesn't care what house they're in, doesn't care if they're dressed in navy or green. A spare is a spare.
Harry returns with Warrington's body and for a moment there's shocked silence throughout the stadium. Except, no one comes to mourn him, there's no horrified wails from his parents as they clutch his cold, unmoving body in their arms. In fact, his parents don't even show up. The Slytherins know why. It's because his parents were Death Eaters. And this strikes fear in the other Slytherins. They begin reaching out to the other houses, utterly terrified because their parents are Death Eaters too.
The man they'd been told would never hurt them because they were purebloods had just harmed one of their own, had harmed an upstanding, pureblood Slytherin without a second thought. The cruel reality that no one was safe from him struck a cruel chord in them all. Voldemort didn't care. And they knew something had to change. This was not like anything they'd planned for their lives, but then again, war was not something you could plan for.
"Life is too unpredictable to plan." That is the claim from a couple of bold Slytherins as they join the DA, signing their names onto the sheet Hermione hands to them without question. They're the first to do so, ignoring the questioning gazes of the other students in the shabby old pub. This is their chance to learn more about Voldemort, without their parents' biased filters. They're shocked when they hear what he's really done. Determined even more to fight alongside Harry, because if one of their own thought him worth protecting, then they would too.
By their sixth year, the Slytherins have branched out and made alliances and friends within each and every other house at Hogwarts. It's too much to take. Being alone, in constant fear of Voldemort - who's been on the rise and growing more powerful for two years now - while their parents just up and leave most days, clutching their arms in pain as they scurry out of the house. It's been two years now since Cassius Warrington, the first pureblooded Slytherin had died at Voldemort's command. And they're finally hitting the breaking point.
Seventh year arrives and it's now a part of the Slytherin motto to get along with other houses, and all are told of Cassius and his sacrifice. The young first years are enlisted right away into the DA, which Neville has taken up running. They learn from day one that Voldemort doesn't care, and it's up to them to protect themselves because he won't. But the other houses will, the Seventh Years assure them of that. So connections grow even further. Slytherin is the house of the determined, they remind themselves, and they're determined to end this war.
And it's in the final battle where the change of Slytherin house is visible to all the world, to all the Death Eaters. The parents of dozens of Slytherins are left shocked behind their masks. But the Slytherins, standing proudly side by side their comrades from other houses, are taking the approach that Voldemort had three years ago when he'd murdered one of their own. So what if these were their parents, or their friend's parents? Voldemort hadn't cared that he'd harmed one of them, and they wouldn't care if they harmed one of his. They had changed, and now they'd fight this war till it was over. They'd fight for the defeat of Voldemort. And most importantly?
They'd fight for Cassius Warrington.