Where do you go if you want to write a story using characters from existing books and TV shows, or even borrowing real people? My 13-year old daughter told me about two popular places: Fan Fiction and Archive of Our Own.
A quick tour of either of these sites will amaze you. You will find hundreds of thousands of stories written by regular folks based on pre-existing characters. Sometimes the store is 100 words long, and other times its 100,000 words. Varying quality, of course, and invitations for feedback.
I had no idea that there were such places.