Well, one possible plot could be that the kidnappers are after some valuable information that Castle has. Maybe it's related to a case he was working on and they think he knows more than he actually does. So they take him to a secret location and try to force him to talk.
Perhaps in this 'castle gets kidnapped fanfiction', it's someone with a personal vendetta against Castle. They've been planning for a long time and finally manage to abduct him. His friends and family then start a frantic search, following various leads that are left behind. And Castle, while being held captive, tries to outwit his captors using his wits and knowledge.
In a 'castle gets kidnapped fanfiction', it could start with Castle being lured into a trap. The kidnappers might be part of an organized crime group that he unknowingly crossed paths with. His disappearance throws everyone into chaos. His partner Beckett is determined to find him and she starts by retracing his steps on the day he was kidnapped. Meanwhile, Castle is trying to find a way to escape from wherever he's being held, which could be an old abandoned building or a secluded cabin in the woods.
The kidnappers could be a group of hired mercenaries. Some powerful and wealthy person who has something to hide might have hired them. This person could be involved in illegal activities and Castle was on the verge of uncovering the truth, so they order the mercenaries to take Castle out of the picture.
Well, in a 'pit gets kidnapped' fanfiction, the plot could involve the search for Pit. Maybe his friends like Palutena start a quest to find him. They might face various obstacles and enemies along the way.
Well, the fanfiction could be that Esme gets kidnapped while she is on her way to an important event. The kidnappers might be holding her for ransom. Her friends then start to look for any clues. They might find out that the kidnappers have a secret hideout in an abandoned building. So they plan a rescue mission, but they have to be careful as the kidnappers are well - armed and dangerous.
It could be that Ginny is kidnapped because she has some special knowledge or power that the kidnappers want to exploit. Her friends form a team to rescue her. Along the way, they discover more about the kidnappers' motives and the true nature of Ginny's importance. There would be elements of mystery, adventure and friendship in the fanfiction.
Well, perhaps Voldemort's followers could kidnap Harry to use him as a bargaining chip. They might hold him in an old, dark wizarding house, guarded by dark magic. Harry would then try to escape using his own magic skills and the help of some unexpected allies like house - elves who secretly oppose Voldemort.
Maybe it's a new villain in the fanfiction. This villain could be after Luigi because he thinks Luigi has some sort of special power or knowledge. So the villain kidnaps Luigi in a very sneaky way, like using a magic spell to put Luigi to sleep first and then whisking him away to a secret lair hidden deep in a dark forest.
In a 'damian gets kidnapped fanfiction', perhaps Damian is kidnapped because he has some special knowledge or power. The kidnappers could be a secret organization. His friends might start to search for him, and along the way, they discover secrets about Damian they never knew before. They have to overcome various obstacles to find him, like decoding mysterious messages left by the kidnappers.
Perhaps Hinata is kidnapped because she has some unique power or knowledge that the kidnappers want. In the fanfiction, the story could focus on how she uses her wits and hidden strength to try to escape while her comrades search for her. For example, she might be able to communicate with a small animal in the cell she's held in and use it to send messages.
One possible plot twist could be that the kidnapper is actually an ally in disguise, trying to protect Mario from a greater threat. For example, they might be taking him away from Bowser's clutches in a way that seems like a kidnapping but is really a rescue mission.
One plot twist could be that the kidnappers turn out to be working for someone Juvia thought was a friend. Another could be that Juvia has a hidden power that she discovers while being held captive, which helps her escape. Or perhaps the person who seems to be the main kidnapper is actually being blackmailed into doing it.