My idea is for a gamemode where darkness kills blockheads, and there's a player that goes around setting "dark" areas and following players with an 8 block "dark" radius. He never comes in contact, he just hovers around. It's like you are an invisible blockhead with a black cloud around you and you just try to kill the blockheads/players by following them surprising them etc.
A more complicated idea is to be able to top down view the map, and click on areas/set areas as "dark areas". So it's like you're playing Whack a Mole but with Players.
