Help needed! Any ideas welcome :)

Hi, all. I need your help. I'm planning to write a script for a Batman short film. I'm good at writing but i can't think of a storyline. Do you have any ideas? I want the main villain to be Joker (and Harley Quinn), and i want Robin and Catwoman to be there too (i hope that's not too much to ask :3) If you can't think of anything Batman related, you can also just leave any ideas for stories whatsoever :) thanks in advance, Luke