Creation of objects by users

You should release within the software a tool to make yourself objects. Regularly, as with maps, compete for the best user-created objects and include them as standard.
The place where items for the competition are uploaded could be a kind of hub outside the game where users can still download the items they didn't win and include them in their standards.
With this feature, the number of items available and their quality would rise exponentially and relieve you of some work.