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.
It could also be that an impostor Spider - Man has been causing trouble, and the Avengers mistake the real one for the fake. The impostor could be doing really bad things like attacking civilians or stealing. The Avengers see Spider - Man at the scene of one of these crimes and immediately attack without realizing it's not the real deal.
One possible reason could be a misunderstanding. Maybe some villains have framed Spider - Man in a way that makes the Avengers believe he's a threat. For example, Mysterio could have created false evidence to make it seem like Spider - Man is involved in some evil plot against the world.
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.
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.
He could be seriously injured. The Avengers are a powerful group, and their combined attack would be hard for Spider - Man to withstand. He might end up with broken bones or other serious physical injuries.
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.
One possible plot could be that the Titans somehow cross into Spider - Man's universe. Spider - Man initially mistakes them for some new kind of giant robots or mutants. He tries to fight them using his web - slinging and acrobatic skills. But he quickly realizes they are much more difficult to defeat as they keep regenerating. Meanwhile, some of the Survey Corps members from Attack on Titan also cross over. They team up with Spider - Man, teaching him about the Titans' weak points at the back of their necks. Together, they fight to keep both universes safe from the Titans.