I first loved the musical when I saw a stage production in the early 1990s, while I was an undergrad. In fact, it may be responsible for my abiding love for a whole genre of stories: side door or back door stories, I call them. I've seen other people call them metafictional or parallel literature.
Whatever you call them, these are stories in which an author/artist/composer, etc. works with familiar material but makes it new by changing the perspective. (Some favorite examples: Wide Sargasso Sea by Jean Rhys, Wicked by Gregory Maguire, Pride and Prejudice and Zombies by Seth Grahame-Smith, "Dolan's Cadillac" by Stephen King, the television series Penny Dreadful).
When this is done well, it's half a love letter to the original material and half something entirely new. Homage and rebuttal, a kind of fan fic which goes down the paths the original piece left unexplored. What could be a better expression of love of a piece of art, than another piece of art inspired by it? I love the way these pieces grapple with the original material, and the interplay between the original and the new version can cast an interesting light on both. When the back door store truly gets to the heart of the original piece, reading (or viewing or listening to) it can feel like finding the original all over again and having the chance to fall in love with it once more.
I've got a few of these kinds of stories sitting on my own back-burners right now that I'm anxious to find the time to write. One riffing on Frankenstein and another on The Turn of the Screw.
What do you think? Are these kinds of pieces derivative? Or just art grown in the garden of other art? Would love to hear your thoughts in the comments below.