Great idea, but there is probably too much legal crap so that could ever work. I mean in theory if a fan creates, lets say raccoon kwami, today and the original show creates another raccoon kwami a even a day later, the fan has a right to sue the creators of so from stealing their character.
Sure the crew could (and should) always ask permission, but then they would probably want to make a written agreement to be sure that the creator won't cause any problems to them in future.
I'm sure that most of people would be honored if the crew would use their character as inspiration, but there is also those few jerks...
The legal complications are a bit more nuanced than that, since the fanmade character is part of a derivative work in the first place. But either way, I'd be very surprised if the show used the ideas of fans without getting permission first. The most likely way I see for something like this to happen would be a competition where fans submit ideas: something similar happened with Vivica, and Doctor Who had an even more similar idea, where a monster created by an elementary school student was featured as a villain in one episode.