I had a thought when I was coming home on the train last week of a storyline. A mermaid/siren seduces a sailor, but then turns into this evil deep-sea fish that drags him down to the depths. She keeps him alive under the water in the depths and slowly things start to grow on him - barnacles, starfish, seaweed, coral, jellyfish etc.
Thinking of doing 3 big drawings of this story and a flipbook to show how she changes and drags him under the water. I dont know whether this is still quite obvious or not, but this is what I have in mind at the moment, and i'm running out of time!