A group of poachers could be the kidnappers. They might think that Serena has information about the location of rare Pokemon in the areas she has traveled. They want to use her to find these Pokemon so they can capture and sell them on the black market.
Perhaps a powerful and corrupted Pokemon could be behind the kidnapping. This Pokemon might be under the control of an evil force or it could have its own motives. It could sense Lillie's special connection to Pokemon and want to use her for its own gain, like increasing its power or finding a way to rule over other Pokemon.
It could be the well - known Team Rocket. They are always up to no good and kidnapping Pokemon for their own gain is right up their alley.
Another option is a random criminal who just sees May as an easy target. He might not even be related to the Pokemon world initially but realizes that he can ransom her to her wealthy Pokemon - loving family or friends. However, he soon finds out that dealing with a Pokemon trainer's friends is not as easy as he thought.
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.