Well, it could be that an enemy of the Avengers and Spider - Man targets a fan as a way to get to them. Maybe this villain wants to use the kidnapped fan as bait to lure Spider - Man into a trap. The Avengers then have to work with Spider - Man to rescue the fan without getting caught in the villain's elaborate schemes.
One possible plot for this 'Avengers Spider - Man Kidnapped Fanfic' could be that a mad scientist who has been observing the Avengers and Spider - Man decides to take a fan hostage. This scientist believes that by studying the fan's reactions and knowledge about the heroes, he can find a weakness in them. Spider - Man, being the friendly neighborhood hero, is immediately alerted. He then contacts the Avengers. Together, they start an investigation. Captain America might lead the strategic planning, Thor could provide the brute force when needed, and Black Widow could gather intelligence. In the end, they manage to rescue the fan and foil the scientist's evil plan.
In a fanfic like this, perhaps a powerful new villain who has a grudge against both the Avengers and Spider - Man discovers a super - fan who has some sort of knowledge or item that is valuable to them. So, they kidnap the fan. Spider - Man might be the first to notice the disappearance as he has a closer connection to the fans on the street level. Then he teams up with the Avengers, each using their unique abilities. For example, Iron Man could use his technology to track the villain, while Spider - Man uses his agility to infiltrate the villain's lair and rescue the fan.
It could be Loki. He's always causing trouble for the Avengers. He might see kidnapping a fan as a way to mess with Spider - Man and the Avengers, since he enjoys chaos and manipulation.
Perhaps there's a new law or regulation in the superhero world that Spider - Man has broken, unknowingly. The Avengers, being the enforcers of sorts, attack him. Like, there's a rule about using certain powers in public and Spider - Man accidentally did it while saving people. The Avengers misinterpret his actions as blatant disregard for the law.
In a fanfic, Spider - Man could be injured while on a mission with the Avengers in outer space. Maybe they are exploring an alien planet and are attacked by some sort of bio - mechanical creatures. Spider - Man is separated from the group and gets severely injured by one of the creatures' acidic attacks. His injury makes it difficult for him to use his powers effectively, and the Avengers have to find a way to both get him back to Earth and find a cure for his injury.
The plot might involve Spider - Man getting into some trouble in New York and the three Avengers coming to his aid. It could be that a powerful alien artifact has been stolen and Spider - Man accidentally gets blamed for it. The Avengers then step in to help clear his name and find the real culprit while also protecting the city from the potential danger of the artifact.
One possible reason could be a misunderstanding. Maybe the Avengers think Spider - Man has been compromised by some sort of enemy influence. For example, if a powerful villain has the ability to control minds and they suspect Spider - Man is under that control, they would try to capture him to protect the world and also to try and break the control. Another reason might be that they believe Spider - Man has some information that is crucial for saving the world but he's being uncooperative, so they resort to capturing him. It could also be that there's some sort of new law or regulation in the superhero world that Spider - Man is unknowingly breaking, and the Avengers, as the enforcers of a sort of superhero code, feel they need to bring him in.
In a fanfiction scenario where the Avengers kidnap Spider - Man, it could be because of a prophecy. The Avengers might believe that Spider - Man will cause great harm in the future according to some ancient prophecy. So, they take him against his will to a secret location. However, Spider - Man manages to convince some of the younger Avengers, like Peter Parker's friends among them, that he's not a threat.
Well, it could start with Spider - Man being framed for a crime he didn't commit. The Avengers believe in his innocence. They fight for him against a corrupt government agency that wants to arrest him. Along the way, they discover that there's a bigger conspiracy at play. Black Widow uses her espionage skills to gather intel, Hulk provides the brute force when they face the agency's armed forces, and the other Avengers contribute in their own ways to clear Spider - Man's name and protect him from harm.
Well, one possibility could be that Spider - Man is mind - controlled and ends up attacking the Avengers. Maybe a villain uses some sort of advanced technology or magic to make him do it. He might be forced to use his web - slinging and super - strength against his allies, causing harm both physically and emotionally to the Avengers and their fans who look up to their unity.
One possibility could be that the Avengers capture Spider - Man because they suspect he has been influenced by some sort of mind - control or dark magic. Maybe they think he's a threat to the city without realizing he's actually trying to help. Another could be that there's a misunderstanding in which Spider - Man's actions are misinterpreted as being against the Avengers' interests.
Well, one possible plot could be that there's a misunderstanding. Maybe some bad guy has framed the Avengers for something really bad, and Spider - Man, believing the false evidence, decides to take them on. He uses his agility and web - slinging skills to surprise the Avengers at first. But as the fight progresses, he starts to realize that something is off and that they might be innocent.