In a fanfiction, Bruce Wayne could be injured in a bomb explosion set by the Joker. This injury not only affects his physical abilities but also his mental state. He has to deal with nightmares and flashbacks of the incident. His friends in the Justice League might come to his aid, but he still has to face his own demons. As he heals, he discovers a new lead on the Joker's whereabouts, which gives him the motivation to fully recover and take revenge.
Maybe Bruce Wayne gets injured while investigating a new criminal organization. His injury is so severe that he starts to question his ability to continue as Batman. This could lead to an internal struggle within him, and also make him re - evaluate his relationships with those close to him. He might even start training a potential successor while he's injured.
One possible storyline could be that Bruce Wayne gets injured during a fight with a new and powerful villain in Gotham. He has to rely on his allies like Alfred and the Bat - family more than ever to keep Gotham safe while he recovers. Another could be that he gets injured in his civilian identity, and has to hide his true nature even more carefully to avoid suspicion as he heals.
Perhaps a story where Bruce Wayne is at the police academy undercover to investigate a series of crimes that seem to be linked to someone in the academy. He has to balance his normal life as Bruce, his training at the academy, and his secret investigation. He could make friends with some cadets who end up helping him in his mission, or he could be in a situation where he has to choose between following the academy rules and doing what he believes is right to solve the case.
One storyline could be that Harry, after the events in the wizarding world, decides to take a break and travels to Gotham. There he meets Bruce, and they end up teaming up to take on a new threat that combines magic and Gotham's usual criminal element. Maybe a villain who has somehow gotten hold of a powerful magical artifact and is using it to wreak havoc in the city.
A possible storyline could be that Percy is on a quest and gets lost in a different dimension which happens to be Gotham. Bruce Wayne, as Batman, finds him and initially suspects him to be a threat. But as they interact, they discover some similarities in their abilities and goals. They then start to dig deeper and find out they are related through a long - lost family connection that dates back to ancient times when the gods had some influence on the early human civilizations.
One possible storyline is a competition between them. They could bet on who can solve a particular global threat faster. Bruce would use his detective skills and resources in Gotham, while Tony would rely on his Stark tech. Another could be that they get framed for a crime together and have to work together to clear their names while also finding the real culprit.
Another storyline could be set during a gala in Gotham. Bruce's girl best friend gets into trouble with some of the seedy characters at the event. Bruce, as Batman, has to save her without revealing his identity to her. This would lead to a lot of internal conflict for Bruce and some interesting interactions between them afterwards.
One possible storyline could be that Coco is injured during a Grimm attack. Her teammates then have to protect her while also trying to find a way to get her the medical help she needs. Maybe they are in a remote area and have to fend off waves of Grimm until help arrives.
One possible storyline could be exploring the immediate aftermath of Bruce's death. How does Terry McGinnis handle the loss? Another could be flashbacks to Bruce's final days, showing his last words of wisdom to Terry.
A really interesting Young Bruce Wayne and Alfred fanfiction storyline could be one where Alfred has a secret past as a spy. This could add a whole new layer to their relationship as he teaches Bruce some of his spy skills. It would be exciting to see Bruce adapt these skills in his crime - fighting later on.
One possible storyline could be that Sakura is injured during a battle. Maybe she's protecting someone and takes a hit that leaves her seriously wounded. Her friends then have to find a way to heal her while also dealing with the threat that caused the injury.
One reason could be that Bruce Wayne never had the traumatic event that led him to become Batman in the original story. Maybe his parents' death was prevented somehow, so he never felt the need for vengeance and justice in that extreme way.