Maybe in the fanfiction, the Avengers need someone with John Wick's particular set of skills for a stealth mission. John Wick initially refuses but is later convinced when he realizes the threat is global. During the mission, they learn from each other's combat styles and strategies, and John Wick's no - nonsense attitude impresses the Avengers.
Perhaps Goku lands in New York City and his power level is sensed by the Avengers. Iron Man is initially skeptical but Captain America is more open - minded. They test Goku's abilities and find out he can move at incredible speeds and has near - invincible defenses. So they ask him to train the Avengers in new combat techniques, which leads to some humorous and action - packed training sessions.
Another possibility is that the Avengers end up in the OHSHC world. The Avengers would be really out of place in that high - school - like setting. Thor might be really confused by all the tea parties and formalities, while Iron Man would probably try to find a way to use the technology there, which is much less advanced than his own. Black Widow could blend in a bit better, using her espionage skills to navigate the social hierarchies of the OHSHC club.
In this fanfiction, perhaps there's a crossover of their skills. Harry Potter could use his magic to enhance John Wick's weapons. For example, making his bullets have magical properties like tracking spells. John Wick could teach Harry Potter how to be more stealthy and efficient in combat without relying solely on magic. The story could be set in a post - Voldemort world where a new danger emerges, and they have to combine their strengths to save both the wizarding world and the normal world that John Wick comes from.
It could work by having a common enemy. For instance, a powerful crime syndicate that has ties to both the Avengers' world (like Hydra having some sort of deal with the syndicate) and John Wick's world. This would force them to collaborate.
A really engaging John Wick fanfiction could center around a time when John Wick loses his memory. He wakes up not knowing who he is or what he's capable of. As he tries to figure things out, he's still being hunted by his old enemies. This would create a lot of tension and mystery, as he has to rediscover his skills and face threats without his usual knowledge and instincts.
A fanfiction could also be based on the Avengers trying to find a way to reverse the Titan transformation. They might see the Titans as victims of some sort of dark magic or science gone wrong. Bruce Banner, with his scientific knowledge, teams up with Armin from Attack on Titan. Armin provides all the information he has about the Titans while Banner tries to come up with a solution. Meanwhile, the rest of the Avengers are fending off any Titans that come their way.
A possible fanfiction could involve a crossover where some of the Avengers are sent to Camp Half - Blood. Imagine Tony Stark trying to figure out how the magic at the camp works while also being highly amused by the whole concept of demigods. Steve Rogers could bond with some of the more heroic demigods over their shared sense of duty. Meanwhile, Percy and his friends have to keep an eye on the Avengers to make sure they don't accidentally disrupt the delicate balance of the camp. And then, they all have to face a threat that targets both the Avengers and the demigods, like a group of villains who want to steal the powers of both.
One idea could be that Naruto is somehow transported to the Marvel universe where the Avengers are. He could use his ninja skills, like his shadow clone jutsu, to confuse the villains that the Avengers are facing. Maybe he teams up with Iron Man and they combine their tech and ninjutsu to create some really cool weapons or defenses.
A fanfiction could involve a crossover where the Avengers are shrunk down to the size of the Gems in Steven Universe. They have to adapt to this new world and its rules. For example, they might have to learn how to use Gem weapons and powers. In return, they could teach the Gems about human combat techniques that are different from what the Gems are used to. This would create a lot of humorous and action - packed situations.
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.