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.
Batman would fit in by being the ultimate strategist. The Avengers sometimes jump into action without a fully thought - out plan. Batman's meticulous planning would force them to be more organized. He could also provide a more grounded perspective. Many of the Avengers have superpowers and deal with large - scale threats from outer space or magic realms. Batman, coming from the streets of Gotham, would remind them of the importance of protecting the everyday people.
It could be really exciting. Batman's detective skills and gadgets would add a new dimension to the Avengers. He might outwit some of the Avengers' enemies with his superior intellect. For example, when facing Loki, Batman could use his knowledge of psychology to manipulate Loki's actions. His utility belt with all its cool gadgets like the grappling hook and batarangs would be a great addition to the team's arsenal.
He might fit in as a sort of mentor figure. He has years of experience fighting crime in Gotham. He could teach the younger Avengers like Spider - Man (if he's in the mix) some valuable lessons about perseverance and the importance of a strong moral code. Also, his physical combat skills would be useful in hand - to - hand battles against powerful foes.
One idea could be that Batman, in his quest to fight crime in Gotham, discovers some sort of interdimensional rift that leads him to the world of The Avengers. He then has to team up with them to stop a common enemy that is using technology from both universes to cause chaos. For example, Loki could be teaming up with some of Batman's rogues gallery like The Riddler to create complex puzzles and illusions that are both magical and technological in nature.
Maybe they're just bored. After all their battles and saving the world, they need some entertainment. Batman fanfiction could be a new and different form of it.
If the Avengers watched Batman fanfiction, they would likely have a range of reactions. Hulk might not care much as he's more interested in smashing things. Black Widow, on the other hand, would analyze the characters and their relationships in the fanfic. Hawkeye might just shake his head at some of the wild ideas in the fanfiction. They would all bring their own unique perspectives based on their own personalities and experiences.
The crossover where Batman takes on the role of a strategist for the Avengers during a major battle against an alien invasion. His tactical mind helps the Avengers in a way they've never experienced before.
If Batman meets the Avengers in fanfiction, it would be a meeting of two very different hero cultures. Batman operates alone for the most part in Gotham, while the Avengers are used to team - work on a global or even universal scale. Batman's gadgets would be a novelty to the Avengers. His grappling hook and batarangs might seem primitive compared to Iron Man's high - tech suit, but they are highly effective in their own right. On the other hand, Batman would be in awe of the Hulk's raw power and Captain America's leadership skills. They could combine their forces to take on threats that neither could handle alone, like a super - intelligent alien race that is both technologically advanced and has super - human abilities.
One possible plot could be that Batman's Gotham is threatened by a powerful villain who has some connection to the Avengers' universe. Maybe Loki decides to cause chaos in Gotham as part of a larger plan against Earth. Batman, being the detective he is, starts to investigate and soon realizes that this is no ordinary criminal. He then somehow gets in touch with the Avengers, perhaps through SHIELD's intelligence network. Together, they combine their skills and resources to take on Loki and his minions. Batman's stealth and strategic mind would complement the Avengers' superpowers.