I almost want to say Anya would make a good Slytherin because of her days as a Vengeance demon and her seeming lack of empathy and/or compassion, but I think she could definitely make a good Hufflepuff. She's tricky. My head canon on HP fusion AUs is vast and somewhat alarming. The amount of time I spent wondering about the Scoobies' patronuses alone is frightening, all by itself.