Monthly contests for player content
Player-created content is tricky. You can't just let everyone make their own hats at will because they might infringe on copyrights or, more likely, make hats out of genitals. At the same time, giving players the opportunity to create content is a major draw for any game.
Once a month have a contest on the forums: make a shirt, make a hat, make a sword, etc. Everyone will submit their submissions on the forums and the mods or Sim or the community will pick/vote on a winner. The item would then be implemented whenever possible.
A few advantages here:
1) All content is screened by the mods and community before it enters the game which would prevent copyright issues
2) It creates a friendly competitive spirit
3) It helps populate the game with more content
Thanks for your time!