I know I don't post here much, but his opic really excited me.
Droosan, I didn't think of Gaudi when Carmen came to mind, but I think your spot on there. I think it could create a nice underlying visual sensuality to base the films style off of Gaudi's organic, undulating style. I could envision such a film having an absolutely stunning color pallete too. Deep dark blues, earthy, dark, red browns and black against these creamy yellows and vibrant reds.
To add to the list I think 'Annie Get Your Gun' could make for an awesome animated film, just so long as it handled the material in a way visualy that could never be done on stage. I think songs like 'Can't Get a Man with a Gun,' and 'Anything you can Do' would be amazing opportunities for animation. They strike me as being a tad like 'The Gaston Song' or 'Be Our Geust' from 'Beauty and the Beast' where you could just visually pull out all the stops. Plus the varied locations of the musical could be really neat visual juxtapositions.
An original animated musical based off of the book version of 'The Phantom of the Opera' would be something I would also love to see, especially since the book features so much wierd, cool visuals. Even more interestng would be a musical animated film based off of Susan Kay's book 'Phantom'. Some editing would have to go on, but I think you could do a lot of that without harming the story. Plus, there would be a great amount of visually stunning locations, (gypsy forests and camps, an opera houses stage and underbelly, a mirror maze, a shah's palace) which could be handled to an amazing effect. I would steer clear of being to similar to Andrew Lloyd Webber's version though, since it is so familiar to audiaces and has so many ardent fans. 'Phantom' based on the Leroux or Kay novels might be a great way to handle such familiar and popular material in a new way, especially Kay's work.