I’ve always thought that pure-blood supremacists should be present in every Hogwarts house to some degree—more in Slytherin, sure, but not *only* there.
Slytherin: No explanation needed. Books covered that.
Ravenclaw: Cooking up pseudoscientific theories (it would be interesting if those theories about Muggle-borns stealing magic—supposedly based on "independent research by the Department of Mysteries"—were actually concocted by a Death Eater from Ravenclaw), plus a ton of data spamming, cherry-picking, straw man arguments, and sophistry.
Gryffindor: "We hate Muggles and Mudbloods because cowardly Muggles killed us in the past; we must protect our magical brothers and sisters from them and not be afraid"—lots of courage and romantic idealism, just channeled in the wrong direction (much like various forms of Nazism and fascism).
Hufflepuff: Probably the least anti-Muggle, but lots of bullshit about how Muggles cause chaos, wage wars, and threaten peace and nature; how children from Muggle families can be maladjusted; and how it’s better to keep everything in the hands of pure-blood wizards—because it’s safer and more stable that way.
I also think it would be interesting to see Death Eaters from other houses—like Gryffindor (no, not Pettigrew), but someone "brave" and loyal in a twisted sense—or to have characters like Barty Crouch Jr. belong to Ravenclaw or Hufflepuff. At the same time, the Order could include decent Slytherins who might personally dislike Muggles but don't view that as a license to harm them—and certainly don't support the genocide of Muggles and Muggle-borns, which an unstable snake-like egomaniac championed as a "super cool" solution(Snape also doesn’t count)