A third storyline could be that they discover a long - lost secret about Gotham's history together. This secret could be related to an ancient order or a hidden power source. As they explore this mystery, they also have to deal with other villains who are also after this secret. Their different perspectives - Batman's more brooding and experienced one and Batgirl's fresh and sometimes more optimistic view - would add depth to their adventure.
Another storyline might involve Batgirl getting into a difficult situation where Batman has to rescue her, and then they train together to make sure she's better prepared for future threats. This could show their mentor - mentee relationship evolving into a more equal partnership. For example, Batman could teach her some advanced combat techniques and Batgirl could share her insights on dealing with certain types of criminals that Batman might not be as familiar with.
A really cool storyline could be a time - travel one in the fanfiction. Batman, Robin, and Batgirl are sent to the past or future Gotham. In this new time period, they have to adapt to different versions of the city and its threats. Robin might be especially fascinated by the differences in technology and crime - fighting methods, and together they work to find a way back to their own time.
Perhaps there could be a storyline where the baby is actually a key to saving Gotham in the future. Batman and Batgirl discover this and have to raise the baby in a way that will prepare it for its important role. They face many challenges along the way, including trying to keep the baby's identity a secret from their enemies.
One interesting Batgirl and Batman fanfiction could be about their teamwork against a new supervillain in Gotham. Maybe a villain who has the power to manipulate time. Batgirl uses her quick thinking and acrobatic skills, while Batman uses his detective abilities to figure out the villain's weakness.
One interesting storyline could be them teaming up to take on a new and powerful villain in Gotham. They use their combined skills, Black Canary's sonic scream and martial arts, and Batgirl's detective skills and acrobatics, to outwit the bad guy.
One interesting storyline could be a reimagining of Batman's first encounter with the Joker in the 1939 fanfiction context. Maybe the Joker starts as a small - time criminal and Batman has to figure out his true motives early on.
Batgirl herself can be a great character in these stories. She's often depicted as a strong, intelligent, and independent woman. Her relationship with Batman, whether it's a friendly partnership or a more complicated emotional bond, is usually a central part of her character development. In some fanfictions, she's written as the one who challenges Batman's ideas and methods, which makes for an interesting dynamic.
In some fanfictions, there might be a story where Red Hood is injured and Batgirl has to take care of him. This could lead to some deep conversations between them about their pasts, their losses, and their hopes for the future of Gotham. It could also be a starting point for a romantic relationship to develop, as they see a different side of each other during this vulnerable time.
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.
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.
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.