Doctor Strange might make an appearance. Since he has knowledge of the multiverse and magic, he could help Peter understand what happened during the snap and how to reverse it. Also, Iron Man could be a part of it. Maybe before he sacrificed himself, he left some sort of clue or technology for Peter to use in this post - snap world.
Obviously, Spider - Man (Peter) himself. And perhaps Aunt May, who would be very relieved that he survived but also worried about the state of the world. Ned could also be there, being Peter's best friend and all. They could be trying to figure out what to do together.
Maybe in the fanfiction, Peter survives the snap but his powers are affected in some strange way. He has to figure out how to control his new - or rather, altered - powers while also coping with a world that has been turned upside down. This could involve him teaming up with some unexpected allies.
Peter Parker would be central. Additionally, Doctor Strange could play a role. Since he has knowledge of the future and different timelines, he might try to intervene and prevent Peter from doing the snap. Then there are the villains. Maybe Mysterio, who has a grudge against Peter, could try to manipulate the situation to make Peter snap in a way that benefits him. And of course, Aunt May would be there in some capacity, perhaps as the voice of reason for Peter.
Thanos could still be the main antagonist. Just because Dr Strange survived doesn't mean Thanos is defeated. He might be on a quest to find out why Dr Strange survived and finish the job. Also, there could be other villains who emerge in the power vacuum left by the snap, and they could team up with Thanos or work against both Thanos and Dr Strange.
One possible plot could be that Dr Strange uses his magic in a way that shields him from the snap's effects. Maybe he creates a temporal or dimensional bubble around himself just in time. He then has to figure out how to reverse the snap while dealing with the chaos that ensued. He might travel through different dimensions to find a solution or seek help from other powerful entities.
There might be new witches and wizards who have heard of his story. They could be either fascinated or disgusted by his past deeds. He may also run into future versions of Hogwarts professors. They would probably be very different from the ones he knew, with new teaching methods and different personalities.
One possible character could be Dr. Connors. Since Peter's parents were also involved in scientific research, he might have had a connection with them. Then there are the people from the Daily Bugle. J. Jonah Jameson could be involved, perhaps with a different view of Peter and his family. There could also be some secret agents or government officials who knew about the Parkers' work and are now interested in Peter because of his parents' legacy.
It's possible that some of Percy's old enemies, like Kronos' minions who somehow survived, might resurface in this fanfiction. They could be trying to stop Percy from achieving whatever goals Chaos has for him. Also, some of the gods might get involved. They could be curious about Percy's new situation, or they might be worried that Percy's new power under Chaos could be a threat to them. And then there could be some mysterious entities from the Chaos - related realms that we haven't heard of before in the original Percy Jackson stories.
Annabeth would be a likely character to appear. Since she and Percy are so close, she might try to find a way to rescue him from Tartarus. Another character could be Nico. He has experience with the Underworld and might be able to offer some help or advice. Grover could also show up, using his nature - related powers to try and sense Percy in Tartarus.
In this fanfiction, Peter Parker doing the snap could be a very different scenario compared to what we saw in the movies. He could be tricked into doing it by a villain who wants to cause chaos. Once he snaps, it might not have the expected results. Instead of simply eradicating half of the life in the universe, it could create some sort of rift or imbalance. Peter then has to find a way to fix this new problem, all while dealing with the guilt of what he has done.
Pepper Potts might appear. She could play a role in helping Tony take care of Peter, like making sure he has a proper diet and a comfortable living space in the Stark Tower. She might also be a maternal figure to Peter, giving him advice on relationships and growing up.