One idea could be exploring Batgirl's internal struggle after the events of 'The Killing Joke'. Maybe she has nightmares and has to face her own fears in a new way. Another could be Batman's reaction to Batgirl's changed state of mind, and how he tries to help her without being too overbearing.
One interesting plotline could be exploring the psychological aftermath for Batgirl after the events in 'The Killing Joke'. Maybe she struggles with PTSD and has to find a way to regain her confidence in the field.
Obviously Batman and Batgirl are the most important. Batman, as the Dark Knight, brings his skills, experience, and his complex relationship with the Joker to the story. Batgirl, having been through the events of 'The Killing Joke', is central to any exploration of the aftermath.
Perhaps a plot where Batman and Batgirl are investigating a series of strange disappearances in Gotham. They find out that it's related to a secret underground organization that is using some sort of new technology. Along the way, they start to realize their feelings for each other are deeper than just being crime - fighting partners. But they have to put their personal feelings aside to stop the organization before it's too late.
Well, fanfiction often takes the themes of 'The Killing Joke' like the thin line between sanity and madness and runs with them. Some interpretations could be about how the events in the story could have been prevented. It might rewrite certain scenes to show different outcomes, like if Batman had reached out to the Joker in a different way before he fully descended into madness. This can offer new perspectives on the characters and the overall story.
Well, obviously Batman and Batgirl are the main ones. Batman is often shown dealing with his complex relationship with the Joker and also his feelings towards Batgirl, especially in the context of the disturbing events of 'The Killing Joke'.
One idea could be a story where Batgirl has to save Batman from a new and powerful villain. Maybe this villain has a special weapon that can neutralize Batman's skills, and Batgirl uses her unique abilities, like her gymnastic skills and her knowledge of technology, to turn the tables.
Another interesting fanfic could be set in an alternate universe. In this universe, Batman is a young rookie and Batgirl is his more experienced mentor. It would be a role - reversal story where Batgirl teaches Batman the ropes of fighting crime in Gotham. This could include her showing him how to use his gadgets more effectively and how to read the criminal underworld better.
One idea could be that Harry, with his magical skills from the wizarding world, somehow gets transported to Gotham City. He teams up with Batgirl to fight a new kind of villain who is using a combination of magic and technology. Harry could use spells like Protego to shield them from attacks, while Batgirl uses her martial arts and gadgets.
Batman: The Killing Joke comic can be valuable. If it's in mint condition and a rare edition, its value can increase significantly. But even common editions have appeal among fans and have a certain worth.
One idea could be that the OC is a new vigilante in Gotham and Batgirl takes them under her wing to train them. This could lead to a lot of action - packed scenes as they fight crime together and the OC learns from Batgirl's experience.