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 Avengers would be very concerned. Captain America would immediately take charge of the situation, trying to keep everyone calm while they assess Spider - Man's injuries. Iron Man would start scanning for the best way to treat him, using his high - tech equipment. Thor might offer some of his Asgardian knowledge of healing, if there was any. Black Widow and Hawkeye would be on guard, making sure there were no further threats while the team focused on Spider - Man.
One interesting storyline could be Spider - Man teaming up with the Avengers to fight a new super - villain that's a combination of different powers they've faced before. For example, a villain with the strength of the Hulk and the mind - control abilities of Loki.
Maybe Spider - Man discovers a new threat that the Avengers have overlooked. He then tries to convince them of the danger, facing skepticism at first but eventually winning them over with his tenacity and evidence. For example, he could find a hidden group of villains planning to use a new type of technology to take over the world.
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 many Avengers fanfics, Spider - Man's sacrifice can be a very powerful plot point. It often shows his selfless nature. He might sacrifice himself to save his fellow Avengers or the world from a great threat. This kind of sacrifice also adds depth to his character as it shows his growth from a young hero to one who is willing to give everything for the greater good.
Iron Man is a great character to pair with Spider - Man. Tony Stark can act as a mentor to Peter Parker, providing him with new and advanced tech for his suit. They also have a fun dynamic, with Stark's sarcasm and Spider - Man's youthful exuberance.
Spider - Man could use his agility and web - slinging skills to escape. He could swing between buildings, making it difficult for the Avengers to follow him. His small size compared to some of the Avengers also gives him an advantage in hiding in tight spaces.