To write a good Batman romance fanfiction, start with a strong concept. Maybe it's a love story that begins during a battle or a chance encounter. Develop the relationship gradually. For example, if it's a romance with a fellow vigilante, show how they bond over their shared goals. Use descriptive language to bring out the emotions. When Batman is in love, his actions and thoughts will be different. You can also add some twists, like a betrayal or a secret that threatens the relationship. And always keep in mind the essence of Batman as a character.