One possible plot could be that Spider - Man gets accidentally transported to the Avengers' headquarters. He has to prove himself to the Avengers at first, since they don't really know him well. Then, they discover a common enemy that is planning to attack both New York City (Spider - Man's territory) and other major cities around the world that the Avengers protect. Together, they combine their powers and skills. Spider - Man's agility and spider - sense work well with Iron Man's technology, Captain America's leadership and combat skills, Thor's godly strength, etc. They have to overcome their differences in fighting styles and strategies to defeat the enemy.
The plot might involve a new super - villain who has the ability to manipulate time. This villain uses this power to change the past in a way that makes both Spider - Man and the Avengers' lives more difficult. Spider - Man realizes what's going on first because of his spider - sense. He then teams up with the Avengers. They travel through time themselves to stop the villain from changing key events. Along the way, there are some cool action scenes like Spider - Man swinging through different time periods and the Avengers using their powers in historical settings.
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.
The Green Goblin could be a great main villain in a crossover. Imagine if he teams up with Doctor Doom from the Fantastic Four universe. Their combined evil intelligence and power would be a huge threat to Spider - Man and whoever he teams up with in the crossover.
Well, it could involve Spider - Man facing a new super - villain. Maybe someone with the power to manipulate time or space, which would really test his skills.
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.
Maybe Spider - Man is trapped in an alternate dimension where his powers don't work properly. The Avengers discover this and use their technology (like Stark's portal - creating tech) and superpowers to break into the dimension and save him. Once saved, Spider - Man joins them on a mission to stop the source of this dimension - trapping problem which could be a new, menacing enemy that the Avengers and Spider - Man have to face together.
One possible plot could be that Spider - Man uses his agility and spider - sense to detect an enemy that the Avengers have overlooked. For example, he might notice some hidden Chitauri technology that is about to activate and cause chaos. So he swings in, warns the Avengers, and then together they figure out how to disable it.
The fanfiction could revolve around Harry Osborn trying to take over Oscorp in a different way. Instead of following his father's footsteps directly, he could use his powers to try and reform the company from within. He might face opposition from the board members and other villains who have their own plans for Oscorp. This would put Harry in a difficult position where he has to use his wits and powers to achieve his goals.