Fred and George would have been in slytherin if Rowling didn’t hate slytherins so much and that’s that on that
Add Percy to the list. Man’s personality revolves around the central Slytherin trait: ambition.
Rather, they would have been in Slytherin if the Sorting Hat sorted based on people’s actual personalities and values more than their choices. Percy was a textbook Slytherin, but he chose to be in Gryffindor because that’s what Weasleys do, and the Sorting Hat honored that choice. Unfortunately, there seem to be a lot of wizards who chose their houses based on family identity, and quite a few others who, like Harry and Hermione, chose their houses based on the houses’ reputations. At this point, “getting Sorted” (rather than sorting yourself, with the hat’s blessing) is probably a rarity – Neville might be the only example we actually meet.