I'm currently working on a novel based on Hans Christian Andersen's original fairy tale, keeping most of the main plot elements the same, but expanding on the story in general adding much darker, more adult themes, like explicit sex and violence, and removing the religious themes of the original story.
The main difference is that the prince and the mermaid genuinely fall passionately in love, and she becomes his mistress and bears him a daughter, but he cannot marry her because she isn't royalty or nobility (as far as they know). So, he promises that she will always have his heart, but marries another woman for political reasons, and she still ends up turning into sea foam, just like the original story.
It's an idea I have not seen anyone else use before, and I think it will make a legitimately good story. Does it seem like an idea I could pitch to a publisher, though?