One reason could be that she feels she has outgrown the team. Maybe she has achieved all she can within the Avengers and wants to explore new challenges on her own. For example, she might want to focus on solo missions that are more in line with her personal moral code.
She could leave because of a falling out with another member. Perhaps there was a conflict over strategy or a personal matter. Like if she disagreed strongly with Tony Stark about how to handle a new threat and it led to an irreparable rift between them.
Perhaps it's due to personal relationships. He might have had a falling - out with one or more of the other Avengers. Like if there were misunderstandings or betrayals. In some fanfictions, a conflict with Steve over leadership or ideological differences could lead to Tony leaving the team.
In some Avengers fanfiction, Natasha's capture might be due to her undercover work going wrong. She is often involved in dangerous espionage missions. If she gets captured, it could be that she was discovered while infiltrating an enemy organization. Her skills are formidable, but no one is perfect, and in the world of fanfiction, this could lead to her capture.
First, they would need to figure out if it was a trap. But assuming it's not, they could divide into teams. One team, like Ant - Man and Wasp, could infiltrate the place in a stealthy way. Meanwhile, Doctor Strange could use his magic to create illusions and misdirect the captors. The Avengers would have to work together, combining their different powers and strategies to rescue Natasha successfully.
One reason could be that he feels a sense of being out of place. Maybe new campers have come in and he no longer feels as important or needed. He might also leave in search of a greater purpose. In some fanfictions, he could be on a quest to find something that he believes is crucial for the safety of the camp, but he has to leave the camp itself to do it.
One reason could be that Naruto might feel he has outgrown Jiraiya's teachings in the fanfiction. He may want to explore his own path independently. Another possibility is that Naruto gets involved in a mission or a situation where he has to part ways with Jiraiya for the greater good. For example, if there is a threat in a far - off land that only Naruto can handle, and Jiraiya has other commitments elsewhere.
One reason could be to seek more power. In some fanfictions, Naruto might feel that the Leaf doesn't offer enough resources or training for him to reach his full potential. So he decides to leave to find a place where he can grow stronger.
One reason could be ideological differences. Maybe she discovers that the Order's methods don't align with her own sense of justice. For example, if the Order starts to use unethical magic in their fight against the Dark forces, Hermione, with her strong moral compass, would not stand for it.
One possible reason could be a difference in their goals. Naruto is from the Hidden Leaf Village with a mission to protect it and promote peace there. Rias, on the other hand, has her own responsibilities in the Underworld. Their conflicting duties might lead to Naruto leaving.
The plot could revolve around Natasha's sister being a secret weapon. She might have been locked away for years because of her uncontrollable powers. When she is finally set free, she has a hard time adjusting to the normal world. The Avengers, led by Natasha, try to train her and help her control her powers. Along the way, they face many challenges, like villains who want to capture her sister for their own gain.
Perhaps in the fanfiction, Sakura had some unique ability that was not accepted or was feared in the village at such a young age. This led to her family deciding to leave the village so that she could grow up without being ostracized or in danger because of this ability.