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.
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.
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.
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.
She could be kidnapped by a villain who wants to use her as leverage against Tony Stark and the Avengers. So the Avengers have to work together to rescue her. During the rescue mission, they discover some new abilities she has that she didn't even know about herself.
Maybe the story could be about her growing up in the shadow of her famous father. She struggles with the expectations of the world and the Avengers. But then she discovers a unique power within herself that has nothing to do with technology. With the help of the remaining Avengers, she learns to control this power and becomes a hero in her own right. For example, she might have a power related to energy manipulation that she initially fears but later embraces.
Well, one possible plot could be Percy using his demigod powers in the world of the Avengers. Since he's Tony's son, he might inherit some of Tony's genius as well. He could be involved in battles, using his sword - fighting skills and water - based powers to aid the Avengers in a new threat that normal Avengers' abilities can't handle easily.
Differences in ideology can also play a part. Tony might have some views on how to handle threats or use technology that the other Avengers don't agree with. Maybe he's more willing to take risks with untested tech that could endanger the team or civilians, which would lead to resentment.
Well, it could be about Tony mentoring Peter. Maybe Tony takes Peter on a special mission to train him. They might face some villains together and during the process, Tony imparts his wisdom and high - tech knowledge to Peter.
Perhaps it's due to personal relationships. He might have had a falling - out with one or more of the other Avengers. Like if there were misunderstandings or betrayals. In some fanfictions, a conflict with Steve over leadership or ideological differences could lead to Tony leaving the team.
One possible reason could be jealousy. Maybe Tony's intelligence and wealth made some Avengers feel inferior, and over time, that festered into betrayal. For example, if Tony developed a new technology that could potentially overshadow their own abilities, they might feel threatened.