That would be legally next to impossible. You would have to clear with those fans compensatory hurdles; licensing or sale of the design outright for use in future ancillary materials such as books, video games, future tv and movie productions, comic books, etc; credit hurdles; clearance hurdles to make sure the designs obtained from fans did not infringe upon any previously copyrighted or trademarked designs. Go look into the Tardigrade lawsuit, or Axanar. CBS can't just reach out and go, "Hey? We're running a little short on time, anyone got any cool ships we can borrow?"