I think this could work for a few different ships, but I'd particularly like some Balthazar/Castiel. Just Balthazar patiently waiting for Castiel? Either to notice Balthazar's attraction to him, or more literally, Balthazar Falling before Castiel and waiting patiently for his brother to join him as a free angel.
I'd really like a portrayal of Balthazar as he is in cannon - snarky, but still incredibly tender when it comes to Cas.