LISTEN TO THE MUSIC
Ideas can form just by listening to music. Many classical pieces, especially symphonic poems, already tell a story. You can use elements from them. I have a plan for a future novel which is based on various parts of a symphonic poem: a hunt, a wedding, and a castle. Or you may imagine something totally different than what the composer had in mind.
This isn’t limited to instrumental pieces. Even song lyrics can spark an idea. So, go ahead. Listen to the music. See where it leads you.