A cool batman crossover fanfic idea might involve a crossover with the Avengers. Batman's strategic mind and detective abilities would be a great addition to the superhero team. He could help Iron Man with tech, or use his stealth to outwit Loki. Also, a crossover with the Doctor from Doctor Who could be fascinating. Batman could travel through time and space with the Doctor, solving mysteries in different eras.
For a batman crossover fanfic, consider a crossover with the Lord of the Rings. Batman could end up in Middle - earth, perhaps using his gadgets to fight orcs and trolls. His detective skills would also be useful in uncovering Sauron's plans. Or, a crossover with the Flash could lead to an interesting story. They could have a race, or team up to stop a villain who can manipulate time.
Maybe in the fanfic, Batman's rogues gallery somehow teams up with some of the Avengers' villains. This forces Batman and the Avengers to work together. Batman could introduce his unique gadgets and combat style to the Avengers, while they could show him their superpowers up close. For example, Iron Man could be intrigued by Batman's utility belt and try to create his own version of some of the gadgets.
Another fanfic concept is that there's a threat in Gotham that has a connection to the Moon Kingdom's past. Sailor Moon travels to Gotham to investigate. There, she meets Batman, who is initially skeptical of her magical powers. But as they start to work together, they realize that their skills are complementary. For example, Sailor Moon's ability to heal could be useful for Batman's injured allies, and Batman's detective skills could help Sailor Moon figure out the mystery behind the new threat.
One idea could be that Batman finds himself in the Marvel universe due to a strange multiverse rift. He teams up with Iron Man to stop a tech - based villain who is stealing advanced technology from both their worlds. Another could be Batman and Spider - Man working together in Gotham. Spider - Man's acrobatics and sense would be a great addition to Batman's detective skills.
One idea could be Batman coming to Star City to help Arrow deal with a new, extremely dangerous criminal organization that has ties to Gotham. This could lead to some great fight scenes as they combine their skills and resources.
One idea could be a story where Batman has to team up with his rogues gallery to fight a greater threat from outer space. Maybe the Joker, Two - Face and the Riddler are forced to put aside their differences when an alien race starts to invade Gotham. They find that Batman's strategic mind and their own unique skills are needed to save the city.
One idea could be exploring Batman's early days as a vigilante in more detail. Show his first encounters with the major villains and how he developed his crime - fighting techniques.
Another story idea is that the Joker somehow gets his hands on a powerful and mischievous Pokemon like Ditto. Ditto can transform into any other Pokemon, and the Joker uses it to create all sorts of mayhem in Gotham. Batman has to figure out how to stop the Joker and his shape - shifting Pokemon while also dealing with other criminals who see this as an opportunity to cause more trouble.
A great plot could involve the two heroes swapping cities for a while. Arrow tries to deal with Gotham's crime underworld while Batman goes to Starling City to investigate a series of strange disappearances that might be linked to a powerful criminal organization there. This would lead to both heroes having to adapt to different environments and crime - fighting styles.
One idea could be that Spider - Man and Batman are both chasing the same villain who has some sort of technology that can disrupt Gotham and New York. Spider - Man's agility and Batman's detective skills would make for an interesting combination as they try to track down the bad guy. Maybe they start off not trusting each other, but as they face challenges together, they learn to work as a team.
A great plot could be that Batman, being the master detective, discovers a plot by some unknown force that is targeting both Gotham and the Avengers' main cities. He contacts the Avengers for help. They initially have some trust issues as Batman is a loner and the Avengers are used to working as a team. But as they start to understand each other's methods, they combine forces. For example, Batman's stealth and strategic skills combined with Captain America's leadership and combat skills could make for some really cool fight scenes against the common enemy.