Mysterio could be the villain. He's all about creating false realities. He could be tricking the Avengers into believing Spider - Man has gone rogue. Maybe he projects false images of Spider - Man attacking the city or stealing important items, which makes the Avengers go after him.
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 be Mysterio. He's known for his illusions and deception. He might create false scenarios that make the Avengers think Peter is a threat. He could manipulate the Avengers' perception of Peter's actions, like making it seem like Peter is causing destruction when he's actually trying to help.
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.
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.
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.
In this crossover, the Avengers might use their high - tech weapons and superpowers to fight the Titans. For example, Iron Man could blast the Titans with his repulsor rays, and Thor could strike them with his lightning. The Titans, on the other hand, are huge and powerful. Their presence would pose a great threat to the Avengers and the world they are protecting. Maybe the Avengers would have to learn about the Titans' weaknesses quickly to stand a chance in this battle.
It could be Mysterio. He's a master of illusions. He might trick the Avengers into believing that Spiderman is a danger, using his illusions to make it seem like Spiderman is doing bad things. So the Avengers, thinking they're doing the right thing, kidnap Spiderman.
In this fanfiction, it might be that Peter has been framed for a crime he didn't commit. The Avengers, who are duty - bound to uphold justice, are misled by false evidence. They attack Peter, but as the story progresses, Peter tries to prove his innocence. He might find some allies within the superhero community who believe in him and help him face the Avengers and clear his name.
A 'Attack on Titan Avengers fanfic' could be really exciting. It might involve the Avengers somehow getting transported to the world of Attack on Titan. Maybe Iron Man could use his technology to fight the Titans, like creating special weapons against them. Captain America could lead the Survey Corps in a battle strategy. And Thor with his hammer could be a powerful force against the Titans as well.
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.