Ryzom

...........

Here's an idea:

An arena where you kill one mob, two more spawn in it's place 5-10 seconds later.

Have it set on a limit though, so it wouldn't go on infinitely.

Also could make it so after X number mobs killed (maybe make x variable?) a boss spawns of said monster type.

Maybe make the respawn on the normal mobs infinite, but keep the x mobs summon boss thing, but make it so that killing the boss ends the scenerio, and you get an uber drop or something.