One possible plot could be that after an incident, perhaps a spell gone wrong or some kind of memory - wiping event related to Tony's tech. Peter Parker is erased from Tony's memory. Peter, who considered Tony a mentor, is heartbroken when he realizes Tony doesn't recognize him. He tries to jog Tony's memory by showing up at the Stark Tower with his Spider - Man suit, but Tony just sees him as a random kid in a strange costume.
Maybe in the fanfiction, Tony has a near - death experience and when he recovers, his memory is jumbled. Peter Parker, who was an important part of Tony's later life, is forgotten. Peter then decides to recreate all the moments they had together, like the time they built the Iron Spider suit. He hopes that by reliving those memories, Tony will remember him again.
In this fanfiction, it could be that a villain targets Tony's relationships. They manage to make Tony forget Peter Parker specifically. Peter is left feeling lost and alone. He starts to leave little reminders for Tony around the city, things that only the two of them would understand, like a specific type of web - pattern that Peter once used to save Tony. Eventually, Peter has to face the fact that he might not be able to get Tony to remember him and has to find his own way without his mentor's guidance.