Well, it could be that an experiment by Doctor Strange goes wrong and the two Peters meet. They are both shocked at first. The MCU Peter is more used to working with a team like the Avengers, while the other Peter has been more of a solo hero. They argue about the best way to fight crime. But then, when a powerful supervillain arrives, they put aside their differences. They combine their knowledge of the city (since they both protect New York in their own ways) and manage to outwit the bad guy.
Maybe in the fanfiction, MCU Peter is on a mission when he accidentally stumbles into a portal and ends up in the world of another Peter. They start comparing their lives, like how different their love interests are. MCU Peter could learn some old - school web - slinging techniques from the other Peter, and they end up having to fight a villain that threatens both of their worlds, leading to a great team - up.
Doc Ock could also be a likely villain. He is intelligent and has those powerful mechanical arms. Imagine the two Peters trying to figure out how to stop his arms from causing destruction. MCU Peter might use his Stark - tech knowledge, while the other Peter could use his more traditional problem - solving skills to take on Doc Ock.
One great fanfiction in this category could be 'The Convergence of Universes'. It explores how MCU characters interact with Earth - 616 Peter Parker in a really exciting way. The story has some great action scenes where Peter shows off his skills from Earth - 616 while also learning from the MCU counterparts.
A more complex plot could involve time - travel. Maybe an enemy from the past is brought to the present and is causing chaos. Peter and Valkyrie are brought together by Doctor Strange or some other powerful character to fix the timeline. They have to travel to different time periods, including Asgard in its prime and Peter's past in Queens. Along the way, they discover secrets about each other and the MCU universe as a whole.
The different power - sets and how they interact. Earth - 616 Peter might have different abilities or a different way of using his powers compared to the MCU version. For example, his web - shooters might work differently or he could have some additional powers that the MCU Peter doesn't. This can create cool fight scenes and problem - solving situations in the fanfiction.
One possible plot could be that Peter, being his curious self from Queens, stumbles upon Thor in a strange situation. Maybe Thor is trying to adapt to life on Earth without his powers for a while. Peter, with his quick thinking and scientific knowledge, tries to help Thor figure out what's wrong. Along the way, they form an unlikely friendship, with Peter constantly asking Thor about Asgard and Thor learning about the modern world from Peter.
In some fanfictions, they might team up to fight a new villain. Picture them combining their powers and skills. Peter Parker's acrobatic skills and spider - sense along with the other Peter's unique abilities could make for an epic showdown against a powerful enemy.
One possible plot could be that Peter Parker, while on a school trip or some scientific research in space (maybe with Stark tech), accidentally stumbles upon Peter Quill and the Guardians of the Galaxy. They initially have a bit of a misunderstanding as their ways of doing things are quite different. Parker is used to the more down - to - earth (literally) problems in New York, while Quill is dealing with intergalactic threats. But they soon realize they can learn from each other. Parker could teach Quill about the latest Earth - based technology, and Quill could show Parker the wonders of the galaxy.
A plot could involve Peter's identity being at risk and Daredevil helping him keep it a secret. Since Daredevil is so good at operating in the shadows, he could teach Peter some new ways to move around the city without being detected. They could also have a conflict about their different approaches to justice. Peter is more about saving everyone, while Daredevil has a more 'punish the guilty' attitude at times.
One interesting storyline could be Peter Parker trying to balance his superhero duties with his relationships in the harem. For example, he has to keep his identity a secret from some of the girls while still being there for them. Another could be the different dynamics between the characters in the harem, like a love - hate relationship with one of them.
One possible plot is that Peter is exploring some strange part of the city, perhaps an alleyway that has some sort of magical anomaly. Loki, who is also drawn to the magic, appears. Peter is initially scared but then realizes Loki might not be there to cause trouble right away. They could end up teaming up to solve the mystery of the strange magic, with Loki teaching Peter some things about magic along the way.