Another possibility is that it's someone related to one of the cases Neal has been involved in. Maybe a family member of a criminal he helped put away. They kidnap Neal to try and get their relative out of prison or to simply punish Neal for his role in the case.
It could be one of Neal's old criminal contacts. Maybe someone he double - crossed in the past and now they're out for revenge. For example, a former forger he worked with who didn't get his share of the profits.
There's a fanfiction titled 'In the Clutches of Danger: Neal'. This one is unique as it not only focuses on Neal's physical situation but also on the relationships that are affected by his kidnapping. For example, how his friends and colleagues react and how it changes their perception of him. Another good one is 'Neal's Captivity', which is a more psychological take on the kidnapping, exploring how Neal copes with being held against his will.
It could be a vengeful criminal that Neal put away in the past. This person has been planning their revenge for a long time and finally sees the opportunity to kidnap Neal.
Perhaps Neal is kidnapped by an underground syndicate that has a vendetta against his old forger friends. They hope to use Neal as bait to draw out those friends. In the fanfiction, it could show Neal's resourcefulness in trying to escape while his friends and the FBI are on a race against time to rescue him. This would involve a lot of twists and turns as the syndicate moves Neal from one hidden location to another.
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.