The plot could start with Batman getting an invitation from SHIELD. They've noticed some strange activities in Gotham that seem related to a threat that could affect the whole world. Batman joins the Avengers. He quickly realizes that their methods are very different from his. But he uses his stealth and detective skills to gather information that the Avengers couldn't. For example, he discovers a hidden base of the villains by following a small clue.
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.
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.
In a 'batman and avengers crossover fanfiction', it could start with Batman being mysteriously transported to the Avengers' universe. He'd be a bit of a loner compared to the team - based Avengers. However, his intelligence would shine as he helps them deal with threats they've never faced before. For instance, against a new type of enemy that can't be defeated by brute force alone. Batman's utility belt and his ability to plan ahead would be crucial in such a crossover. He might even teach the Avengers a thing or two about being more strategic in their battles.
The plot could involve Percy Jackson's world of Greek gods colliding with the Avengers' universe. Maybe the gods have some sort of issue that only the Avengers' powers can help with, or vice versa. For example, an ancient Greek monster has been unleashed in the modern world that the Avengers are struggling to defeat, and Percy and his friends come in with their knowledge of Greek mythology to assist. It could also be a situation where the Avengers are dealing with a threat that has its roots in Greek magic, and they need Percy's demigod powers to counter it.
The plot could involve the ghouls from Tokyo Ghoul being brought into the Avengers' world. Maybe the Avengers initially mistake the ghouls for new, dangerous threats. But as they learn more about the ghouls' situation, they might try to find a way to co - exist. For example, Kaneki could share his story with the Avengers, and they could be sympathetic. Then they could team up to fight a common enemy, like a new type of super - villain who wants to use the power of ghouls for evil purposes.
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.
The plot might involve a common threat that forces the two groups to team up. For example, a new super - villain who has the ability to steal quirks and use them against the heroes, while also having technology similar to what the bad guys in the Avengers face. The Avengers would be impressed by the unique quirks of the My Hero Academia characters, and they'd have to figure out a way to combine their skills to defeat the enemy.
How about a plot where the Joker teams up with one of Spider - Man's villains, say, the Green Goblin? They plan to create a new type of toxin that can drive people insane and also give them super - strength. The toxin starts to spread in both Gotham and New York. Batman and Spider - Man start to investigate separately at first. But as they realize the connection between the two villains and their plan, they join forces. Batman uses his intellect to analyze the toxin and find an antidote, while Spider - Man uses his speed and webs to stop the villains from spreading more of it. In the end, they manage to save both cities and put the villains behind bars.
The plot might center around a new super - weapon. In the Halo universe, there are powerful weapons like the Halo rings. Imagine if some evil force in the Marvel universe gets hold of this technology. The Avengers and Halo characters would have to work together to stop it. For example, Captain America's leadership skills and the Chief's military tactics would be crucial in formulating a plan to retrieve or destroy the weapon.
The plot might involve the Avengers being called to investigate strange energy signatures which turn out to be related to the AllSpark from the Transformers universe. The Decepticons are after the AllSpark and the Autobots are trying to protect it. The Avengers and Autobots join forces. Thor could use his powers to fight off the Decepticons' attacks from the air, while Hulk could take on the physically strong ones. The fanfiction could also explore how the different personalities of the Avengers and Transformers interact, like the witty banter between Spider - Man and Bumblebee.