The kidnapper could be a power - hungry villain from another dimension. This villain has heard of Dr Flug's reputation and thinks that by kidnapping him, he can gain access to advanced technology. Or it could be a group of bounty hunters who were hired by someone who has a grudge against Dr Flug. They would capture him and deliver him to the person who hired them.
Maybe it's a cult leader. The cult might believe that Reid has some sort of special knowledge or power that they want to harness. They kidnap him to convert him to their beliefs or use his mind for their own strange purposes.
One possible plot could be that Dr Flug is kidnapped by a rival villain who wants to steal his inventions. Another plot might involve Dr Flug being kidnapped by an organization that wants him to work for them against his will. Or perhaps he's kidnapped by someone who has a vendetta against his boss and is using him as bait.
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.