Use the tiles which the people in OOO use to build user generated levels, which can be submitted to OOO for an exorbitant about of cr and CE or can be used on a guild private gate, which is controlled by the guild master - this again is submitted for a high level of crowns.

NOTE: Crowns are placed automatically with an algorithm, items are limited by a "heat bar" which works effectively the same way as the LBP thermometer (Each item added fills up the heat bar, and it cannot be allowed to overflow), enemies have a counter limit to it so that you cant get a level full of enemy or crown spam.

A similar system is used on transformice - which works very well