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.
Perhaps a secret organization that has been observing Tony's work for a long time. They realize the potential of his technology and decide to kidnap him to force him to work for them exclusively. Or it could be a time - traveling enemy from the future who knows how important Tony will be in future events and tries to stop him by kidnapping him in the present.
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 terrorist group. They might see Tony as a valuable target because of his wealth and technology.
The Avengers would be in a state of panic and fury. Captain America would likely lead the charge to find Tony. He'd use his military strategy skills to plan a rescue mission. Thor might use his godly powers to track down any signs of Tony's whereabouts. Black Widow and Hawkeye would be on the lookout for any Hydra agents or informants. Hulk could be a wildcard. He might be uncontrollably angry and ready to smash anything in his path to get to Tony.
One plot could be that a powerful enemy kidnaps Tony Stark to steal his Iron Man technology. They hold him in a secret lair and try to force him to reveal all the secrets of his suits. Another could be a fan with an extreme obsession who kidnaps him, thinking they can make him see things their way. Maybe they want him to create a special weapon just for them.
Another possibility is some disgruntled former employees. Maybe they feel they were wronged by Stark and want to get revenge. They know his routines and weaknesses well enough to plan a successful kidnapping.
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.