I second Tristan & Isolde. This is the only one that came to mind and it’s really not new. 
The Lone Ranger and Tonto Fist Fight in Heaven and The Gods Must Have Blood would be great titles to put on a marquee.
My favorite idea would be a Catcher in the Rye adaptation called Holden! The Musical
When I was in London a couple years ago I saw a marquee for ‘Hard Times: The Musical’, so I guess anything’s possible.
Pride and Prejudice has also been done as a terrible, tanked musical called ‘First Impressions’ (which might have been classed as an operetta, but I don’t think so).
I nominate Coleridge’s unfinished (or do you think it was finished? where are the Romantics out there?) poem ‘Christabel.’ Totally creepy, but with happy upbeat songs about the father/daughter/happy home, etc., love songs about the absent lover . . . then the creepy stuff. With someone like a younger Elaine Page as Geraldine.
For composer I have no idea. I might nominate a cross between the composers of Jekyll and Hyde (the London version, before people started changing it) and Ragtime.
There’s a French musical of that (Notre-Dame de Paris). It’s kind of weird and probably really bad if you ask anyone with taste, but I love it anyway. I have the DVD. There was an English translation, and it played in Las Vegas for a while, but the translation was horrid.
I’m pretty sure there was a Tale of Two Cities musical, but it never got anywhere. From what I’ve heard of it, it was quite derivative of Les Miz.
Lolita.
No, I’m serious. I think if you got just the right mix of serious and comic, it might work as a musical. Or be the most spectacular trainwreck Broadway has ever seen. Either way, well worth the money.