I'm in the process of writing a musical with my best friend and now collaborator, and we have another noe on the shelf for when we're done. Both are based on books, and I have gotten the permission of both authors. The simple answer is, I don't think you can just sit down and start to write a musical, because no matter how much composing or writing talent you have, if you haven't got a good story that sings then you don't have anything.
Really, you can't just fish for ideas, when the right story comes along you'll know. I read the book to the first of my two musicals five years ago, it only hit me in February that it would make a great musical. I believe that if the story isn't really important to you or if it's just an idea somebody threw at you then it'll be an uphill battle to write it as a musical, because I really think your heart has to be in it.
I hope you find the story you're looking for, and good luck!
The rain we knew is a thing of the past -
deep-delving, dark, deliberate you would say
browsing on spire and bogland; but today
our sky-blue slates are steaming in the sun,
our yachts tinkling and dancing in the bay
like racehorses. We contemplate at last
shining windows, a future forbidden to no one.
Derek Mahon
"Maybe all one can do is hope to end up with the right regrets."
Arthur Miller