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.
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.
One idea could be that Tony is kidnapped by a group of his tech rivals who want to steal his latest Iron Man suit designs. They hold him in a secret underground lair. Another could be that a fan-turned - villain kidnaps Tony because they think he didn't do enough to save a certain character in the Avengers' battles. Or maybe a powerful alien race kidnaps Tony to force the Avengers to give up some powerful artifact they possess.
In some fanfictions, Tony might be kicked out of the Avengers because of a conflict in leadership styles. Maybe his high - tech and sometimes risky solutions don't sit well with other members who prefer more traditional or cautious approaches. For example, if a mission goes wrong due to one of his untested inventions, the team could lose faith in his decision - making abilities.
Yes, there are likely fanfictions out there with such a plot. Fanfiction writers often explore different dark and angsty scenarios for their favorite characters. However, I can't specifically point you to one as there are countless fanfictions in the Avengers fandom.
Maybe it's his wealth. His vast wealth can create a sense of inequality within the team. Some Avengers might feel that he uses his money to overshadow them or make decisions based on his financial power rather than the best interests of the group. Also, his high - tech gadgets and suits might make other members feel inferior or left out.