The Weasleys could have been great Slytherins.
Arthur might've been a great Hufflepuff, too, and Charlie and Bill do make for good Gryffindors, but just think of them all in green and silver—think of those sly eyes, the coy curls of their lips, each one. If Ronald Weasley wasn't meant to be a hero, they all would have worn green.
It would've clashed horribly with their hair but, well.
See, Molly Prewett Weasley was always sharp, quick as a whip and harsh as one, too, and she would've been the epitome of a Slytherin Head Girl. She's not a troublemaker, won't go looking for a fight, but she'd never held fear enough not to stop one, and she was always a mother at heart. Her Slytherin first-years asked questions to the Prefects first, but she stepped between them and their tormentors first, too, and there was always a reason Ginny Weasley would cast a powerful Bat-Bogey Hex. It's genetic.
Molly is-was-will be a spitfire, a clever, dastardly woman. She'd slide you the ripest fruit at lunch, but she'd snag some for herself, too, and she knew how to use the title "pureblood" to take care of others. Blood traitor she may be, but an arse she was not, because she skewered everyone who spoke down to the muggleborns, the half-bloods, Slytherin or not. She was a Slytherin as they were meant to be: confident, sly, unafraid, and with a healthy love of reciprocation. There is a difference between a lack of fear and a helping of courage, and she was not courageous like the Gryffindors would be. There is a difference between vengeance and returning what was given, and every hex she learned was out of necessity, not rage.
Arthur Weasley, the first Weasley in Slytherin in near one hundred years (or so he liked to claim, as a point of pride), was one of those people that could never be a House poster-child. Like Neville Longbottom wasn't the cookie cutter lion, he wasn't the cookie cutter snake his wife was. Neville Longbottom was sorted Gryffindor because he needed to understand what he could become, in spite of his grandmother's expectations. Arthur Weasley was sorted Slytherin because there was something he needed to become: more assertive. More like a rock, steady, because for all that Slytherin is not its stereotypes, those stereotypes do have basis, and Arthur always had a fascination for muggle technology. One can imagine what that kind of fascination meant for you in Slytherin.
See, Slytherin was meant to serveArthur in the same way Molly was meant to serve Slytherin, and neither of them came out lesser for it. Arthur Weasley, sorted Slytherin, learned how to deflect, redirect, and—in dire moments—cut through, and he became a shield for the younger kids in the same way Molly was their blade. He, blood traitor, muggle-lover, pushover (though he wasn't, really), stood against the tide and came out of it with young Slytherins starry-eyed, thinking that's my green and silver. That's my perfect Slytherin. They're the same kids that latched onto Molly's sleeves, learned to talk to her before the Prefects, let her fight their battles because they knew otherwise they'd lose.
That's probably why Molly Prewett ended up Molly Weasley. Because they, both of them, were blood-traitors, defenders of the Better Slytherins, parents in their own right. That's probably why they ended up with so many kids, too.
William "Bill" Weasley was the first of their kids to grace their old haunts, and the first to take up the mantle Molly had left behind—Prefect, then Head Boy, a defender of the little ones, though not in the same way. Molly is-was-will be a spitfire, quick to snap and lash with her tongue and her wand. Bill was cooler than that; he knew how to play the long game, how to make fear sneak into the minds of young, dumb Slytherin blood supremacists. His game was subtle in a way his parents' never was, which endeared him just slightly more to the other Slytherins. He was still a blood traitor, of course, and freakish for the punk casual clothes he'd wear on weekends, but at least he could follow a conversation in the way it was really being spoken.
Bill was good at catching the whispered insults and correcting them, had the most level, intimidating stare, but he also knew how to let loose. He was never the strict kind of Prefect his brother Percy ended up being; he didn't report the parties, the underage drinking, the magical experiments. He'd wink and steal a swig, instead, and he wore his shark-tooth earring long before Molly ever caught wind of it. But no one could deny that Bill had the quick hands of a true Slytherin when wands pointed at muggleborns slipped from pureblood's grasps and into his nimble grip, when dangerous potion ingredients tossed toward neglectful Gryffindors' cauldrons were snatched from the air, when his hand caught the wrist of a punch about to fly and squeezed, his eyes narrowed, his voice cold. Bill Weasley knew how to give a kid nightmares when it mattered.
Charles "Charlie" Weasley was next, the first of their Quidditch prodigies. Star Seeker, Captain of the team, and Prefect; a full resume for a blood traitor in the most prejudiced possible house. Charlie was, maybe, one of the more mellow Weasley kids, but he was never less Slytherin for it. How could one otherwise account for all the times he stole away to the Forbidden Forest, to meet and study all those fascinating creatures that lived there? He was ambitious in that he knew what he wanted to shoot for and did, and he was sneaky in that even Bill didn't know how often he broke the rules to moonlight in the forest.
Maybe it was wrong, but it was well within the capability of the Slytherin team Captain to reject Charlie. They could've spat at his feet, called him foul things, taunted him, never even let his broom into the air. Despite that, they did. Charlie Weasley, a reprehensible, traitorous fiend, ended up Seeker, because his Captain was more interested in having a good team than hating muggles. The worst kept secret in the Burrow is that every Weasley hates their first year, before they learn to cope with the venom, and this was the moment that Charlie knew things would work out: when his skill in the air mattered more than anything else, when his Captain shook his hand and meant it.
Charlie Weasley was more like his dad than his mum, when it comes to Slytherin, but it was the Molly in him that knew how to use "Seeker" against "pureblood." Few people would mouth off to a Quidditch player, even the littlest of them. Everyone knows they can pack a punch, and in Gryffindor that might've meant a black eye, but in Slytherin—well. Charlie was never against using his encyclopedic creature knowledge to more personal ends.
The third of them was Percy Weasley, and it was stamped across him, that word ambition. He was goal incarnate, rigidity in human form, and this was the Prefect, Head Boy perfect the professors always wanted. Percy was cold enough to fit the same scary Slytherin profile every other house saw, even if he was laughed at with derision in his own common room (staid, serious Percival, who'd ever take him seriously?). The Slytherins always put on a united front in the corridors, and Percy was their epitome: cold, unnerving, gaze cast down on the younger years. As much as Percy was foolish and endearing in the eyes of his brothers, he was prone to snapping at the firsties, too, and he was quick to take points. The Sorting Hat said "Slytherin" for him nearly as fast as it might for a Malfoy.
Percy got a perfect set of OWLs, eyes set firmly on the Ministry, and despite the jeering he endured—or perhaps because of it—he was the steadiest of his family. He could take the most verbal punches. He never stopped moving forward, and it was probably this that foreshadowed the day he'd disown them. He was ready to do anything to get where he needed to, and casting off the label "blood traitor" ended up being a necessity in his eyes. (It wouldn't have been, really, if he'd thought a little harder, but.)
Then were Fred and George, and they were ambition like Percy was, like Charlie was. They knew what they wanted and went for it, and if they were more sneaky than cruel, that didn't quite matter. Slytherin has "sly" in the name, after all, and they knew every hidden passage. The Fred and George that wore green were a tad harsher than the ones who wore red, their experiments a touch more dangerous, but they still valued a laugh over all else. They were always charismatic, no matter the house, and that helped them when they had to talk circles around Slytherin's blood purists.
The worst kept secret of the Weasley family is that the Weasleys hate their first year, then learn to tolerate their fellow house members, and find joy in other things. Fred and George Weasley are the first of them who learn to love Slytherin, wholeheartedly, and regard the bad apples as individuals, separate from the core of what Slytherin is. Fred and George are the ones who coax stories from muggleborn and half-blood lips, are the ones to wrap their arms around the younger year's shoulders and take them with, rather than let them go on alone after chasing the mean ones away. They were best friends with Lee Jordan in all universes, but in this one, they gained a fair following of sweet, dastardly Slytherins, too, and none of them skipped out on buying from Weasley's Wizard Wheezes.
(Ron comes next, but we'll get to him in a moment.)
Ginevra "Ginny" Weasley was both the least and most Slytherin of them all. She was ruthless, terrifying, a whirlwind, an infinite spinning top. She grew into her confidence, and Slytherin taught it to her: what it meant to be assertive, to stand on your own two feet. She had six older brothers and a thirst to prove herself, and that meant that every step she took was in effort to stand apart from them, stand individual. She was not a Weasley blood traitor bitch but Ginny, spitfire like her mother but ruthless on her own, and by the end of her stint at Hogwarts, her narrowed eyes and powerful casting carved her name the way she meant it. Even at the end of her first year she had a reputation, and Percy, who habitually looked out for all their siblings, had stopped even trying to help her. She didn't need it.
See, first-year Ginny Weasley was a little girl for the first half, a bitch and then a threat for the second, and second-year Ginny Weasley was retroactively dubbed the true Heir of Slytherin. It was a secret that the diary had possessed her, so obviously everyone knew, and Harry was always a come-and-go target. She hated the label, at first, but came to relish it: the nervous glances people threw her way, the hesitance in even just the new first-years to speak to her. She didn't like being feared but she despised being babied, and it was with a purely Slytherin enjoyment that she encouraged the label: Ginny Weasley, Heir of Slytherin, ruthless.
In this universe, Ronald Weasley wasn't meant to be a hero. It just so happened that he ended up becoming one, and because he was not destined for it, his family was one of Slytherins, and when he met Harry Potter on the train, and Hermione Granger (whirlwind girl like Ginny would be, just a little braver) stuck her head into their compartment and talked about Houses, and Harry asked him where he thought he'd be, he said, "Well, my family's all been in Slytherin, so I 'spect there. What about you?"
And Harry wrinkled his nose a little, ignoring his question, saying "I met a boy named Malfoy in Madam Malkin's. He said he'd be going into Slytherin, too. He seemed like a right prick." Ron snorted into his hand with laughter.
"You'd be right about that. Dad complains about his dad all the time, they know each other from work. Listen, there are some gits in Slytherin, but it's not all bad. It's got a bad reputation, but my family's all right," Ron said, with a pride (a love) for them that he has in all universes.
And Harry, having known him for a few hours, looked him up and down and said, "I think you're right."
Ronald "Ron" Weasley" had a lot to live up to: six brothers, one sister, and not a unique desire in him. He liked Quidditch, but Charlie was their star Seeker, and he was smart enough to make good marks but not as good as Percy. Ron wanted-needed-asked for more, and he was sorted Slytherin because that's what the green and silver was good for—more.
And, because of Ron Weasley, when the Sorting Hat nudged Harry Potter toward Slytherin, he looked at the scattered redheads dressed smart in silver and green and, instead of thinking Not Slytherin, he thought, Anywhere's good, but Slytherin I'd like.