First, understand the characters well. Read the original comics or watch the movies to get a feel for Batman's stoicism and the Riddler's eccentricity. Then, create a unique plot. Maybe start with a new riddle that the Riddler poses that has far - reaching consequences. Build the relationship between them gradually, whether it's a friendship, a rivalry, or something more. Also, use vivid descriptions of Gotham to set the mood.
Writing a good Batman Riddler slash fanfiction starts with research. Know the backstories of both characters, their motives, and their goals. Then, come up with a central conflict that is engaging. It could be a moral dilemma that they both face. In terms of their relationship, explore the different aspects of it. Is it a love - hate relationship? A mentor - mentee relationship in disguise? As for the writing style, be descriptive but not overly wordy. Let the dialogue flow naturally, as this is crucial for bringing out the personalities of Batman and the Riddler. And finally, add some twists and turns to keep the readers on their toes.
To write a great Batman Riddler slash fanfiction, you need to focus on character development. Show how the two characters change over time due to their interactions. For the plot, think of something that challenges both Batman and the Riddler in new ways. It could be a new villain in Gotham who uses their own weaknesses against them. When it comes to their relationship, don't rush it. Let it develop organically, perhaps starting with mutual suspicion and slowly evolving into something deeper. And don't forget to add some action scenes that are true to the Batman universe.