it's very inconvenient to give users the ability to upload content that was created outside of your game (i assume you're speaking of .unity3d files and not simple images, and unity3d bundles can't even hold script files). you'd need:
depending on how complex your content / level is, the more challenging the normalization gets.
answered Jun 09 '11 at 11:46 PM
I think you might be better off to create a kind of level creator within the game. But even that would be very difficult to implement.
answered Jun 10 '11 at 12:09 AM