I have a suggestion for a Private Server. I didn't get to try this yet since I'm having trouble installing a patch. How about a building and when someone enters it there's two elevators. The elevators lead to a ceiling creating a glitch that will cause the player to be pushed through the ceiling and be trapped at the top. (I'm not sure if this will work.) One leads to a middle of a stadium with a high level mob of some sort. The other one leads to benches right near the stadium with invisible walls. Through the invisible walls players can see the other player fight the mob. If the player defeats the mob it will drop a reward and an item that will teleport them out of the stadium and will be destroyed after 1 use. The people who were watching can just walk down a stairs to exit.