Maybe she's kidnapped by a group that wants to use her as a bargaining chip. For example, they could be trying to exchange her for some valuable technology or access to a secure location. Natasha would then have to use her wits and combat skills to not only free herself but also foil their plans. It could also be that she's kidnapped by a new super - villain who wants to prove themselves by taking on a powerful Avenger like Natasha.
One possible plotline could be that Natasha is kidnapped by a rival organization seeking to extract information from her about S.H.I.E.L.D. secrets. They might try various forms of interrogation, but Natasha, being the tough spy she is, manages to hold out and start planning her escape. Another could be that she's kidnapped by someone with a personal vendetta against her past as a Black Widow, perhaps a family member of someone she killed during her assassin days. They want to make her pay for her past actions.
A possible plotline could be them having to keep their marriage a secret from the world. Maybe because of Tony's business rivals or Natasha's undercover work. They would have to navigate their relationship while hiding it from everyone, which could lead to some really tense and exciting moments.
Maybe she's a congresswoman fighting for the rights of superhumans or former spies. She could be in the middle of political intrigue, where her past secrets are at risk of being exposed and she has to use her wits to keep them hidden while still doing her job in Congress.
Another plot could involve Harry Potter being sent to the Marvel universe where he meets Natasha Romanoff. Harry, with his knowledge of magic, could assist the Avengers in a threat that has a magical or otherworldly origin. Maybe they have to deal with a dark wizard from Harry's world who has found a way to enter the Marvel universe.
It could be a disgruntled former S.H.I.E.L.D. agent. Maybe they felt betrayed by Natasha or S.H.I.E.L.D. in some way and decided to take revenge by kidnapping her. Another option is a new villain in the Marvel universe who wants to make a name for themselves by taking on one of the Avengers.
A third plotline might involve a situation where one of them is in danger, and the other has to go to great lengths to save them. For instance, Natasha gets captured by a powerful enemy. Clint then uses his tracking skills and knowledge of Natasha's whereabouts to mount a solo rescue mission. Along the way, he reminisces about all their past missions and how much Natasha means to him.
In a fanfic, there could be a plot where Natasha is poisoned and the only antidote can be created by Bruce using his knowledge of biochemistry. But he has to do it while keeping the Hulk in check. As he races against time, Natasha's condition worsens. This creates a tense situation that tests their relationship and shows how much they mean to each other. They also share some tender moments during this difficult time.
One interesting plotline could be a mentor - mentee relationship. Natasha, with her vast experience, could be mentoring Peter in the ways of espionage and more covert superhero work. This would involve training sessions where she teaches him how to gather intelligence and be more stealthy. Another plotline could be a story where they are both on a mission to save a group of hostages. They have to rely on each other's skills to get through various obstacles and enemies.
One possible plotline could be that Keith is kidnapped by an enemy force. His friends then embark on a dangerous rescue mission, facing various challenges along the way. Maybe they have to infiltrate a heavily guarded enemy base to find and free him.
One possible plotline could be that Naruko is kidnapped by a powerful enemy who wants to use her as a bargaining chip against her friends or family. Another could be that she is kidnapped by a mysterious organization for some sort of experiment. Maybe she's kidnapped by someone who has a personal vendetta against her and plans to make her suffer.
Perhaps in this 'imprijt kidnapped fanfiction', the kidnapped person is a key figure in a grander scheme. Imprijt kidnaps them to use as a bargaining chip against their friends or allies. The friends then embark on a rescue mission filled with various challenges and unexpected twists.