One possible scenario in such a fanfiction could be that Peter, in a self - sacrificing move, takes on a powerful enemy that was originally meant to target Tony. Maybe Peter's inexperience and pure heart lead him to misjudge the danger, and he ends up giving his life to save others, while Tony is left to grapple with the guilt.
One possible plot development could be that Peter is left with a huge sense of guilt. He might struggle with survivor's guilt, constantly thinking that it should have been him. He could then become overly reckless in his superhero activities, trying to prove that he was worthy of being saved. Another aspect could be how the other Avengers deal with Tony's death. They may turn to Peter as a symbol of hope, since Tony sacrificed himself for Peter. This added pressure on Peter could lead to him either rising to the occasion or crumbling under it.
The story could focus on Tony's downward spiral into guilt and self - blame. He might isolate himself from the other Avengers, feeling that he should have been the one to die. His technological advancements could take a dark turn as he tries to find a way to bring Peter back, perhaps even dabbling in dangerous and unethical science.
Peter's character arc would take a very dark turn. He'd likely be consumed by grief and a sense of obligation. His once - carefree and enthusiastic approach to being a superhero would be replaced with a more solemn and determined one. He might become more secretive, not wanting to let others in on his pain.
Well, clearly Peter Parker and Tony Stark are the main characters. Peter is the one who 'snaps' in this scenario, which means he has a significant character shift from his normal self. Tony, on the other hand, serves as a sort of contrast or the catalyst for Peter's snapping. Even though Peter is the one with the new behavior, Tony's presence and actions are likely to be integral to the story as they are what cause Peter to snap.
In this fanfiction, it could be that Peter snaps after Tony makes a decision that Peter strongly disagrees with. Let's say Tony decides to sacrifice a certain group of people to save the majority in a moral dilemma. Peter, who has a stronger sense of justice for the individual, snaps and decides to go against Tony's plan. He then has to fight not only the external threat but also deal with the rift he has created within the team.
The 'tony is dusted instead of peter fanfiction' could have a very interesting plot. Since Tony is such a central and resource - rich character, his sudden dusting would be a huge shock. Pepper would be devastated. Peter, on the other hand, would be in a difficult position. He might try to use his own powers and intelligence to figure out a way to bring Tony back. The other Avengers would be looking to him for leadership in some ways, as he was the one closest to Tony among the younger heroes. And the whole dynamic of the post - 'dusted' world would be completely different from what we saw in the movies, with new alliances and strategies being formed in the absence of Tony.
You can try looking on some popular fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfictions, and there's a good chance you might find it there.
It could also be that some of Thanos' followers or those who try to take advantage of the post - snap chaos become the main antagonists. For example, some of the rogue elements in the universe might see this as an opportunity to gain power and territory, and they would oppose the Avengers who are trying to restore order. So they could step up as the main threats in the fanfiction.
Well, one possible change could be that the Avengers' dynamic is severely disrupted. Tony was a key figure in terms of resources and leadership. Without him, the remaining Avengers might struggle more in their fight against Thanos. Peter, on the other hand, being young and full of potential, his presence could have added a different kind of energy to the post - snap world. Maybe there would be more of an emphasis on the younger generation of heroes stepping up earlier.