The more I've thought about it, the more I'm convinced that you could make a DAMN good story out of an Ursula+Triton prequel story. Triton is already an antagonistic figure for most of The Little Mermaid so making Ursula seem a bit sympathetic in the past with him as a foil isn't a stretch. Seeing details of how Triton came to hate humans or when Ursula crossed the line that got her banished... There's real potential there. Why the hell were Maleficent and Cruella the first villains Disney thought of to try to give a sympathetic backstory to?