Here Comes Mr. Jordan/Heaven Can Wait (Although Damn Yankees is close enough!)
I'd like to see any of the Coen Brothers movies musicalized, especially Fargo or Raising Arizona.
I think quite a few of John Irving's books could be musicalized (his characters are so quirky and detailed). The World According to Garp and The Cider House Rules (among others) have already been made as movies, but I think the plot for Until I Find You, with its lead character being an actor, might be a better choice for a musical.