In some fanfiction concepts, Batman might die in a cross - universe conflict. For example, if there are portals opening up to other dangerous universes in Justice League Unlimited. He ventures into one of these dangerous realms to stop a threat from coming into their universe and ends up getting killed. His death could then be a catalyst for the Justice League to find a way to permanently close these portals and prevent further incursions.
One possible storyline could be that Batman sacrifices himself to save the world from a powerful alien threat. He uses his intellect to devise a plan that requires him to give his life. Another could be that he is poisoned by a new villain in the Justice League Unlimited universe, and despite the efforts of his friends, he succumbs to the poison.
Another storyline might involve Batman leaving because of a personal crisis. For example, he discovers something dark about his past that makes him question his place in the Justice League. He might think that he's a liability to the team and decide to leave to protect them.
The Flash might initially be in denial. He's used to Batman always having a plan and his death would be a huge shock. He would then turn his speed into a tool for investigation, trying to find out exactly what happened. Green Lantern could feel a sense of loss but also be spurred into action, using his ring to try and prevent future tragedies like Batman's death. He might create constructs in Batman's memory to inspire the team.
Perhaps there's a story where Batman joins the Justice League just in time to help prevent an alien invasion. The League is struggling against a technologically advanced alien race, and Batman's strategic mind is the key to formulating a plan. He uses his resources and knowledge of Earth - based technology to create counter - measures against the alien weapons. Along the way, he forms bonds with his new teammates, especially with Wonder Woman, who respects his tenacity and intelligence. This experience makes him more integrated into the Justice League and changes his perspective on the larger scope of protecting the world.
One possible storyline could be that Batman discovers a conspiracy within the Justice League that endangers Gotham. So he and Robin decide to take on the League to stop it. Another could be that the Justice League is being mind - controlled, and Batman and Robin are the only ones who realize it and try to free their friends by fighting them. Or perhaps there's a misunderstanding where the Justice League believes Batman and Robin have turned bad, and they have to fight to prove their innocence.
One possible storyline could be Kid Batman trying to prove himself to the full - fledged Justice League members. He might be given a small mission that seems simple but turns out to be full of unexpected challenges, like dealing with a new villain who has powers that can nullify the League's usual tactics. Another could be Kid Batman forming an unlikely alliance with a less - known member of the Justice League, and together they uncover a conspiracy within the League itself. Maybe there's a story where Kid Batman has to use his wits to save the day when all the other members are incapacitated.
One popular storyline could be a time - travel adventure. Imagine Batman Beyond getting accidentally sent back in time to the era of the original Justice League. He has to team up with the younger versions of the heroes to stop a threat that could change the future. For example, a villain from his time could have followed him back and is now trying to rewrite history to gain ultimate power.
One interesting storyline could be the Justice League having to deal with a mind - controlled Batman. Maybe some villain manages to take over Batman's mind, and the League has to figure out how to stop him without harming their friend. It would be full of action as they try to outwit Batman's strategic plans while also trying to break the mind control.
A third possible storyline could be a story of misunderstanding at first. The two groups could initially see each other as threats. For instance, the Justice League might mistake the Z - fighters' high power levels as a sign of aggression. However, through a series of events and battles, they come to realize that they are on the same side and must work together to face greater threats. This could involve some epic one - on - one battles between characters like Vegeta and Superman before they finally team up.
Perhaps Ghost Rider has a conflict with some of the more morally upright members of the Justice League at first. He operates in a more gray area, and they might not trust his methods. But over time, they come to respect his power and his sense of justice. For example, when dealing with a villain who has a connection to the occult, Ghost Rider's knowledge and abilities prove invaluable, and he earns their trust.
In the Justice League Dark and Batman story, one main storyline often involves the exploration of dark magic and the supernatural. Batman, being a detective, tries to understand and combat these otherworldly threats using his wits. Another aspect could be the interaction between the members of Justice League Dark, like Zatanna and Constantine, and Batman, as they have different approaches to dealing with the dark forces.