Hats most likely take a lot of resources to make so this wont happen.
You can already make your own custom cape if you are on PC
(and this has already been suggested)
Costumes also can take resources to make. You can make a character in the minecraft skin customizer but it won’t show up on hive (for a good reason of course) (
)
You can already make zombie arms on PC but idk if Hive could even do this. They could make more costumes with arms like that perhaps but I don’t see them doing this with normal skins.
Welcome to the forums tho 