I like the Arena match system thats in place where people can use all spells...etc available to them, but not a lot of people use it. I'm not sure of your network architecture or whether players jump between physical servers for various dungeon instances currently, but would it be possible to have an Arena instance/s which players from all servers can enter?
You could host it on one of your least utilised servers.
There's a strong pvp community on each server, but at any particular time of the day its not extremely large. This could make for some epic team v team battles.