I think a good item to add for back piece skins would be existing items like the shields. It would just be the skin for the back piece. I believe most everything to do this is already in the game and it wouldn’t require new art.
Just a little something to keep customizing our characters.
If you really want to go crazy, perhaps a mini creature hanging on to your back/shoulder like a wizard’s familiar? =) That’s a lot of work though.