BoiGeorge
A crazy idea but.. WHAT IF Merlin was a reincarnator as well? That actually sounds like a good plot for a fic A goofy mc names himself merlin when he's reborn in hp world just for fun. In his travels he helps a kind boy pull out a sword from a stone that he saw because the boy needed a weapon, the boy then introduces himself as arthur. Then the mc is struck with a realization MC: "Wait... am I.. merlin?" He keeps denying it, he just wants to tour the world and experiment magic. He then sees arthur and his friends put their swords atop a round table MC: "NOPE this is just a coincidence. Still not merlin!" He meets morgana.. MC: "NOOOOO"
I think you are venturing into AU territory with making Slytherin malicious. He was into black magic, true, but he was ultimately a decent person with some prejudice. At least according to the source material. He did offer up his family home and lands as a school for children, so I find it hard to believe he was a bad person.