Heh. I reinterpreted the story of Perseus and Medusa as "Perseus has to do the same thing, but without any supernatural help - just gods that bug him all the time." =P I wouldn't call that fan-fiction.
Let's be clear: Shakespeare's Romeo and Juliet is not original by any means. It is based on a long narrative poem by Arthur Brooke titled The Tragicall Historye of Romeus and Juliet, published in 1562, and and retold in prose in Palace of Pleasure by William Painter in 1567. Shakespeare expanded the stories, especially the parts of Mercutio and Paris.
Brooke's poem was itself a translation of Italian stories by Luigi da Porta and Matteo Bandello, and a French version by Boaistuau also influenced the narrative.
The original Italian versions, and potentially Brooke's, were influenced by the ancient Greek myth of Pyramus and Thisbe, as told by Ovid in his Metamorphoses, which also features a story of forbidden love and tragic deaths. (Pyramus and Thisbe are quite possibly the originals). There is also Ephesiaca by Xenophon of Ephesus, written in the 3rd century, which contains several similarities to the play, including the separation of the lovers, and a potion that induces a deathlike sleep. (See here for more: Romeo and Juliet (Sources) - Wikipedia).
Shakespeare very skillfully used poetic dramatic structure, including effects such as switching between comedy and tragedy to heighten tension, the expansion of minor characters, and numerous sub-plots to embellish the story). He certainly had plenty of dramatic skill. But to call Romeo and Juliet original is to ignore the debt he owes to the previous authors.
Anyway, all this mention of romantic duos reminds me of one of my favourite songs ...
I'm sorry I brought the damn thing up in the first place.