Everyone's read a different version of the original Hans Christian Anderson story, and while the details vary from version to version, the dark nature of the tale remains the same. In short, the original story is a lot like the Disney movie, but the Prince marries a land princess instead and Ariel dies from grief. Sony recently acquired the film rights for the novel Mermaid: A Twist on the Classic Tale by Carolyn Turgeon. This story takes a very new approach, with the rival princess as the protagonist attempting to marry the fish-loving Prince to bring peace to her kingdom. Neither woman is particularly keen on giving up the Prince, and his choice will cause a political dilemma either way.
The whole darker-side-of-princess-stories thing is being done to death right now, but the premise is interesting enough. Turgeon has also written a Cinderella retelling, focusing on the later years of the failed Fairy Godmother. Both books seem to have quite the following, so there's hope for decent source material here.