There's a possibility that it's a super - villain who has been waiting for the right moment to strike. Tony being in Siberia gives the villain a chance to isolate him from his usual support system. This villain could be someone with a vendetta against Tony, perhaps someone whose plans were thwarted by Tony in the past and now wants revenge by kidnapping him.
It could be a terrorist group. They might see Tony as a valuable target because of his wealth and technology.
Perhaps in this fanfiction, Tony is kidnapped from Siberia by a powerful villain who has a long - standing grudge against him. The kidnapper could be after Tony's Iron Man suit technology. They snatch him from Siberia as it was a place where he thought he could be safe or was conducting some top - secret research related to his suit's power source. His friends then have to embark on a dangerous rescue mission.
It could be Hydra. They've always been after Tony's technology and his influence. Maybe they see an opportunity to get an edge over the Avengers by kidnapping him.
It could be a disgruntled former employee. Maybe someone who felt underappreciated by Stark Industries and decided to take revenge by kidnapping Tony. Another possibility is a new super - villain who wants to make a name for themselves by taking on Iron Man. They see kidnapping him as the perfect way to get attention.
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.