As an author, you have the freedom to make variations on age-old themes. You can make your own adaptation of them, or you continue where the original ended. Lots of bestselling authors have done this, like for instance Teresa Medeiros, who made her own version of some fairy tales. The three novels Charming the Prince, The Bride and the Beast and A Kiss to Remember are obviously based on Cinderella, Beauty and the Beast and Sleeping Beauty.
Right now, I'm stuck with my WIP (happens sometimes) and then I know I have to find something else to keep me occupied. Short stories work marvels for it, or novellas. Once this short work is finished, I usually find the inspiration to continue on the WIP and get it also finished.
Because I so like Phantom of the Opera, and also have read the book by Gaston Leroux, I am playing with the idea of making my own phanom story. Nothing to do with Andrew Lloyd-Webber's musical (although it will feature in the story line) and also nothing borrowed from the work of Leroux.
No, it will be my own version of a phantom story, set in the Parisian opera house. Because at the time this building was constructed, there was an urban legend about a ghost and about a mysterious lake beneath the theater. So why not place my own ghost in this decor? And a beautiful girl, of course. After all, it has to remain a romantic story, and one with a happy ending.
I like happy endings. In my books the girls always gets her man, her true love. Just fiction, something to escape from when life is bad.