Alot of people love the PVP in minecraft, and a good arena is something this server doesn't have. You have a few mini-games now but many of them can only be played during server events. What if a few people want to duel it out? They can make bets against each other and it would be a fair fight, not who has the best enchanted armor. Anyway here is something I built yesterday that I thought would be good to kick start this idea.
It is a four player archery battle with platforms to jump across for a close quarters battle. But it comes at no risk. Miss a jump or get hit by an arrow while jumping across mean certain death with lava lurking in the depths.
And here is the schematic link.
http://www.planetminecraft.com/project/archery-wars/And you can't really tell in the pics, but the lava is about 10 blocks below the structures so nothing will catch on fire randomly.