Well, it could start with Mycroft being lured into a trap and kidnapped. The kidnappers might be from a different dimension or have some sort of supernatural power. As the fanfiction progresses, Mycroft could use his wits to outsmart the kidnappers even while in captivity. There could be a lot of mind games and tense moments as he tries to escape or signal for help.
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.
One possible plotline could be that Stiles' friends immediately start a search. They might follow clues like strange footprints or a mysterious note left behind. Another could be that Stiles manages to outwit his kidnappers in some way while being held captive, perhaps by tricking them into revealing their weaknesses. Maybe the kidnappers are after something Stiles knows, like a secret about the supernatural world in the fanfiction's setting.
One possible plotline could be that Yami is kidnapped by a rival group who wants to use his special powers for their own evil deeds. His friends then embark on a dangerous mission to rescue him. Another could be that Yami is kidnapped as part of a ransom plot against someone he is close to, and they have to figure out how to meet the kidnappers' demands without sacrificing too much.
One possible plotline could be Harry using Mycroft's deductive skills to figure out the true nature of his parents' death earlier. He might not trust Dumbledore as blindly and instead start his own investigations.
One possible plotline could be that a deranged fan kidnaps the Beatles in an attempt to force them to play a private concert just for them. Another could be that a rival music group arranges the kidnapping to try and stop the Beatles' rise to stardom.
One possible plotline could be that Aizawa is kidnapped by a group of villains who want to use his quirk knowledge against the heroes. They might hold him in a secret lair and try to force him to reveal the weaknesses of other heroes. Another plot could be that a deranged fan kidnaps him, believing that they can have a special relationship with him. The fan might keep him in a locked room, trying to convince him to like them back.
One possible plotline could be that Twilight Sparkle is kidnapped by a powerful dark magic user who wants to steal her knowledge of magic. Her friends then embark on a quest to rescue her, facing various challenges along the way. Another could be that she is kidnapped by an unknown force from another dimension for some sort of experiment. Her friends have to figure out how to cross dimensions to save her. A third plotline might be that a group of villains kidnap Twilight Sparkle to use her as a bargaining chip against Equestria, and her friends must find a way to outwit the villains and get her back safely.
In some PLL Emily kidnapped fanfiction, the story could be centered around a misunderstanding. Let's say there's a new group in town that mistakes Emily for someone else who they think has wronged them. So they kidnap her. As she's held captive, Emily tries to convince them of their mistake. Meanwhile, her friends are worried sick and are trying to find her. They search through her recent activities, her contacts, and any strange happenings in town. They might also get some help from Caleb or Toby, who have their own skills in finding people. Emily, on her part, might find a way to communicate with the outside world, like using a hidden phone or leaving signs in the room where she's being held.