I suppose it depends if this is going to be explicitly as written in the books Slytherin, which as I recall is basically a bunch of posh British asshole elitists; or fanon NotAllSlytherins!Slytherin/misunderstood!Slytherin. Or something in between, which is what I'm hoping for.
I hope that the QM, regardless of whatever fanon or headcanon he may rely on, finds a way to rationalize the setting enough to feel like an immersive world we can freely move in.
Like, even without relying on any NotAllSlytherins OC character, fact is that ~1 in 4 wizards who grew up in Magical Britain did in fact graduate from Slytherin. So the nicest Slytherins can't possibly be worse than the bottom ~25% of the population. And Magical Britain is still functional in some way or another despite being made up of ~25% Slytherins.
I know HPMoR gets hated on around here (often enough for good reason), but I like its explanation for Slytherin House. To summarize as I understand it, the Sorting Hat doesn't force students into houses they don't want to go to, but at the same time tries to keep an even number of students in the four Houses if possible. It also balances what is good for the student currently under the Hat with what is good for all students in general.
Back in the day, Salazar was the only Founder who didn't want to teach muggleborn at Hogwarts. But keep in mind that all of the Founders were exceptional people and that their moral stances and opinions weren't really reflective of the average. Salazar's bigotry was much more mainstream and normal back then and in no way his defining or even an exceptional trait. Probably even the other Founders wanted to teach muggleborn out of the goodness of their hearts and not because they thought (let alone knew) that they were equal to trueborn wizards.
Over time though, as racism against muggleborn declined and became less and less widespread, more and more wizard families that clung to that bigotry (which morphed more and more into hatred as these things do when minorities move from being an "obviously inferior" curiosity to a politically significant demographic that can be blamed for things) aligned themselves with their image of Slytherin. And so on the flip side more and more young wizards came under the Hat with thoughts like "anything but Slytherin", even if they were otherwise cunning and ambitious and charismatic and pure blooded. And so, over the last century or so Slytherin had fewer and fewer wizards who would have been a good fit overall and more and more wizards who could have also fit somewhere else, but who stood out for either agreeing or not having a problem with anti-muggleborn bigotry, until it ended up in the sorry state of being Tom Riddle's prime recruiting ground and in the aftermath of that only became worse.
Yeah, well, Dudley'll be sorted before Harry, sooo...the latter might end up in Hufflepuff or Ravenclaw. Which I'd find a pretty nice butterfly, to be honest.
Harry's primary reasons for not wanting to go to Slytherin (if I remember correctly and am not confusing canon with fanon) was that he had two rather negative encounters with Draco Malfoy and Hermione's exposition on the four Houses. Both of those are things that could get butterflied away, and if Dudley becomes a Gryffindor on top of that, Slytherin does become a decent possibility.
I also vaguely remember that Hermione was a good fit for three of the Houses and chose Gryffindor in part due to her experience with Harry, Ron and Neville on the Hogwarts Express. And Granger and Potter both come before Weasley (a Gryffindor shoe-in). In other words, I think the whole event is ripe for being majorly influenced by butterflies.