Hmm, you could check whether there's anything already on one of the fanfic archives like http://archiveofourown.org/ ... or you could search DW for who has an interest in Phantom and see if any of them write fic... phantomoftheopera might be a good place to start (you could join and post something introducing yourself and asking whether anyone knows of fic that matches what you're looking for). Hope you find it!