Allow players to submit graphics to you for possible implementation in-game.
You have a game where the player creates the world, why not allow the players to enjoy another aspect of game making other than level/map creation. They could submit enemies, different player outfits (which could be sold for plat), and blocks. Then you could take what you want and trash what is useless.
As for the legal aspects, if you have legal rights to the game itself, then isn't it just a matter of agreement when players submit content which will or will not be used in your game?
Would ruin the game! Sorry, bad idea!