Griddlehark selkie AU would be such a vibe. The themes of how forcing someone to stay with you makes them hate you. How loyalty and commitment have to be voluntary.
If Harrow is the one that stole Gideon’s coat, it would work best if they were small children. Harrow doesn’t want Gideon to ever leave her alone, and she heard that if you take their coat, a selkie can never leave. It would be a sadder and angrier take on a Ponyo sort of story, two small children learning how to (and how not to) form bonds and make connections.