You have to REALLY define for us "original." That will determine the criteria.
A CHORUS LINE, in my opinion, should count. Those interviews were field research for the show, so it was NOT based on existing material.
To me, an ORIGINAL musical is one that is not based on an existing property (ie novel, film, TV show, comic book, fairy tale, biography, news story, etc.).
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle