Well, he might go through the day feeling really sad and neglected. Maybe he'd see other people celebrating birthdays and it would make him feel even worse. He could start doubting his relationships with his friends and family.
In a fanfiction like this, it could be that Peter's girlfriend (if he has one in the story) forgets his birthday. She's been so busy with her own work or studies. Peter is heartbroken, but instead of getting angry, he uses the time to work on some new Spider - Man gadgets. Eventually, his girlfriend realizes her mistake and she arranges a really romantic and private celebration for him, strengthening their relationship.
One idea could be that all of Peter's friends are so caught up in fighting crime and their own lives that they completely forget his birthday. Peter feels really down about it at first but then realizes that his real purpose is to be Spider - Man and save the city, not focus on a birthday celebration.
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.
Peter might get invited to the Avengers' headquarters. There, he could see all their cool gadgets and technology. He would be like a kid in a candy store. The Avengers could ask him for advice on how to deal with street - level threats, since he has more experience in that area. Peter would be really happy to contribute and feel like he's part of the team.
In such a fanfiction, Peter might lose his usual control. He could accidentally reveal his identity while drunk. For example, he might start talking about his Spider - Man adventures without realizing it.
In a 'Peter Parker meets Shuri' fanfiction, they could team up to fight a common enemy. Maybe a new super - villain emerges in both their worlds and they combine their unique skills. Peter with his spider - abilities and Shuri with her brilliant Wakandan technology.
Well, he might become extremely powerful. He could use the power of the gauntlet to protect his city in ways he never could before. Maybe he could even defeat the most powerful villains with a single snap.
He could be using his phone to track a villain's movements. Maybe he installed some special app on his phone that can detect unusual energy signatures. So while the teacher is droning on about history or math, Peter is secretly following the path of a super - villain who is on the loose in the city.
If Peter Parker joins the Avengers in fanfiction, it would change the team dynamics significantly. Peter is used to operating on his own in Queens, but with the Avengers, he'd have to learn to work as part of a much larger and more powerful team. He could introduce the Avengers to his street - level knowledge, which might be useful when dealing with villains who operate in the shadows. His relationship with Aunt May could also play a role, as the Avengers might need to protect her from any potential threats. Plus, his growth as a hero would be accelerated by being around such experienced and powerful teammates.
Perhaps Peter Parker gets invited to the White House for some sort of science fair or young innovators' event. There, he meets the president and they strike up an unlikely friendship. The president could then call on Peter for help in situations that require his unique skills, like dealing with a super - villain who has some connection to national security.
Well, Deadpool might crack a lot of jokes and make Peter Parker super confused. He could also try to team up with Peter to fight some villains, but his crazy methods would probably drive Peter nuts.