Have you ever played the IOS game Heroes and Castles? I'd assume not. Anyway, it is basically a castle defense game. So, my idea was that you could have a castle defense minigame where mobs spawn in waves and try to kill you. Using 1.9 mechanics and some plugins, you could create something unique. I can't think of any Minecraft servers that have combined idea of a mob arena and castle defense. What do you think of this idea?