Since you are saying that: OP posted about Warframe. Warframe has a system in place called Tennogen, where fans and modders are allowed to create their own skins and share them with the Warframe developer. Good stuff then gets implemented in the game. Sure, that would require quite a bit of lawyer work to set such a system up without risk of legal liability, but it would not be impossible to allow fan contributions in their game (after a quality check, of course).