(for expanding something already in process)
Visualize the world of the thing you are writing: its geographic center, its horizons. Then imagine a figure who could know about that world, perhaps from far in the future of your story, perhaps by virtue of reports or surveillance, perhaps because of magic omniscience or prescience. Write a ballad for that figure to sing, that gives a history of the world of your story. Set it wherever you want in time and space. Borrow the tune of an old ballad or a corrido and invent new words. Use the exercise to let the song tell you something you didn’t know about your story.