With the addition of dueling arenas, I will bring up this ideas that I had. The point is simple. Two giant groups from each house will duel at the same time. The house remaining moves on. Whoever wins the finale will get some house points. This will encourage teamwork, competitive nature, and house bonding. They will be balanced because some players know lots of spells, some only a couple on both sides. It also tests people's aim because you don't want to accidentally hit your teammate or heal your enemy, would you?
I was thinking
Raven vs Gryffindor
Syltherin vs Hufflepuff
Winner of Round 1 vs Winner of Round 2
But it can be anyway you'd like.