The kidnapper in a Cinderella kidnapped fanfiction might be a spurned suitor of the prince. This person is angry that the prince chose Cinderella over them and decides to take her away. It could also be a long - lost relative of Cinderella's stepmother who has come to town and wants to cause trouble for Cinderella. Additionally, it could be a mysterious stranger who has some ulterior motive related to the magical elements in Cinderella's story, like her glass slippers or the magic of the fairy godmother.
Well, it could be an envious maid from the palace. She sees Cinderella getting all the love and attention from the prince and decides to kidnap her. Maybe she thinks that if Cinderella is out of the picture, she can win the prince's heart. Or perhaps it's a dark wizard who has been observing Cinderella from afar. He wants to use her pure heart for some dark magic ritual, so he kidnaps her. There's also a chance that it could be a group of outlaws who heard about Cinderella's new wealth and status and think they can get a big ransom for her.
It could be an old enemy of Keith's, someone from his past who has a vendetta against him. Maybe a former rival from his days in the military academy who has now turned to more devious means.
In some creative fanfiction, it could be an alternate version of Spiderman himself from a different dimension. This Spiderman has gone bad and kidnaps a fan from our dimension as part of some grand, dark plan that involves crossing dimensions and causing chaos.
A power - hungry nobleman from a neighboring land could be the kidnapper. He might believe that by having Rapunzel in his possession, he can gain some sort of political advantage or use her as a bargaining chip against her kingdom.
It could be Orochimaru. He's always been interested in obtaining powerful individuals for his experiments, and Naruko with her unique abilities would be a prime target.
It could be a new villain that has emerged, someone who is jealous of Gohan's power and wants to make it his own. This villain might be from a far - off planet and has heard of the Saiyan legend.
It could be a group of rogue Decepticons who are acting on their own. They might not follow Megatron's orders but still see value in kidnapping Arcee. Maybe they have a personal vendetta against her or think they can get rich by selling her to the highest bidder in the Cybertronian underworld.
Maybe it's a group of villains working together. They could be from different factions but united by the idea that Izuku has the potential to become a huge threat to their plans. They might kidnap him to either eliminate him or turn him to their side. Also, it could be a lone, obsessed fan of a villain who wants to prove themselves by bringing Izuku to their idol.
A group of outcasts from different islands might kidnap Moana. They feel that they have been wronged by the 'normal' society that Moana represents. They hope that by kidnapping her, they can get the attention they deserve and force the islands to change their ways. But Moana listens to their grievances and tries to find a way to help them while also finding a way home.
It could be Zeref. Since Zeref has an interest in powerful magic users and Natsu is a strong dragon slayer, Zeref might kidnap Natsu to study his magic or use him in some dark plan related to his quest for immortality or power.
It could be a jealous rival from his own village. Maybe someone who is envious of Shikadai's natural talents and popularity, so they decide to kidnap him to get him out of the way.