It could be Loki. He might have some new trick up his sleeve where he makes the Avengers forget Tony as part of a bigger scheme to cause chaos among the team and weaken their defenses. He could be planning to take over the world while the Avengers are distracted by this strange memory loss.
One possible plot could be that Tony makes a great sacrifice during a battle but the Avengers, due to some mind - altering spell or extreme shock, forget all about him. Tony wakes up alone, injured, and has to figure out how to deal with his new situation while also trying to find a way to make the Avengers remember him again.
In a fanfiction scenario, it could be part of a plot to create some drama. Maybe there's a villain who has a mind - control device and made them forget on purpose. This would lead to Tony feeling hurt and left out, which could then drive the story forward as the Avengers try to make it up to him and also deal with the threat of the villain.
Thanos could also be a good option. Imagine Bella's planet holds one of the Infinity Stones that Thanos needs. He would stop at nothing to get it, and Bella and the Avengers would have to join forces to protect it. Thanos' brute strength and strategic mind would pose a huge challenge for them.
A powerful necromancer could be a great main villain. He tries to control the ghostly avengers for his own evil purposes, using dark magic to manipulate them.
There could be a new, powerful alien being as the main villain. This alien might have the ability to influence Hulk's mind and make him knot Tony. The alien's goal could be to weaken the Avengers by taking out Tony, who is a key strategist and tech - support for the team. The fanfiction could then follow the Avengers as they try to figure out how to defeat this alien and free Tony from the knot.
Maybe it could be a time - traveling criminal. This villain has the ability to jump between different timelines and has noticed that by disrupting the key events in both the Arrow and Avengers universes, he can create chaos and then rule over the resulting mess. He would constantly be one step ahead of the heroes as he could manipulate time to his advantage. He could also use his knowledge of future events to set traps for the Avengers and the Arrow team.
Tony would probably be really hurt. He's a human after all, despite being a genius billionaire playboy philanthropist. Birthdays are special, and for his friends to forget would sting.
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.
Maybe an old enemy from Wakanda's past. This enemy sees the deaged Tony and T'Challa as easy targets. He wants to steal Wakanda's technology and Iron Man's tech. He tries to manipulate the deaged heroes and use them for his own evil plans.
It could be Mysterio. He's always been jealous of Peter's connection to Tony. He uses his illusions to mess with Tony's mind and make him forget Peter. Mysterio wants to be the one in the spotlight and sees Peter as an obstacle, so by making Tony forget him, he can get rid of Peter's influence on Iron Man.