This is a quick variant I came up with the other day.
It assumes you are using the "Duelling Wizards" extra rule.
Each house of Hogwarts has a particular homework they favour. If a player of a house has collected the appropriate homework then they will add 1 to their rolls when in a wizard duel.
Slytherin: Potions (Snape being the head of the house and all!) Ravenclaw: Library (Value learning and intelligence) Hufflepuff: Care of Magical Creatures (Have an affinity with living things) Gryffindor: Divination (Great destinies and all that!)