Perhaps it's a dark wizard who escaped from Azkaban and blames the Weasley family for his downfall. Ron being a Weasley makes him an easy target for revenge. The kidnapper could have some dark and elaborate plan to use Ron as a pawn in a bigger scheme against the wizarding world.
It could be a disgruntled Slytherin student who is jealous of Ron's relationship with Harry. This student might think that by kidnapping Ron, they can get to Harry and cause some chaos at Hogwarts.
It could be a Death Eater who escaped justice. They might see Ginny as a way to get revenge on the Weasley family or as a means to gain power. Since the Weasleys were always on the side of the good guys against Voldemort, a remaining Death Eater could target Ginny.
One possible plot could be that Ron is kidnapped by a dark wizard who wants to use him as bait to lure Harry Potter. Maybe this dark wizard believes that Harry would do anything to save his best friend. Another idea is that Ron is kidnapped by a group of rogue house - elves who are angry at the wizarding world's treatment of them and think that taking a prominent wizard like Ron will make a statement. Or perhaps he is kidnapped by a fanatical fan of a rival Quidditch team who wants to disrupt the Gryffindor team's performance.
It could be a new, powerful villain who wants to make a name for themselves by taking out two well - known sidekicks. This villain might have some unique powers or abilities that make it difficult for Robin and Kid Flash to escape.
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.