Every hidden service I've been to ends .onion, but I haven't any technical explanation as to why that is.
I've also never found any information regarding whether or not it is possible to host a hidden service with any TLD other than .onion.
I've tried changing my hostname file to mysite6dfas3ds65.onion to mysite6dfas3ds65.potato, however it reverts to .onion after restarting tor.