The doppelganger could be a clone of Peter created by some mad scientist. The clone has Peter's memories up to a certain point but then has been programmed with different motives. So when they meet, Peter has to figure out how to deal with this exact copy of himself that has been turned into a tool by someone else.
In the 'Peter meets his doppelganger fanfiction', perhaps the doppelganger has a different personality. Peter is friendly and outgoing, but his doppelganger is more reserved. This causes a lot of misunderstandings at first. But as the story progresses, they discover that they have some common goals, like protecting their neighborhood from a new threat. So they put aside their differences and work together.
Peter Parker is the main focus, of course. His parents are central as well. Depending on the fanfiction, villains could also be important characters. If Peter's parents have some sort of unfinished business with a villain, it could lead to a big showdown. And don't forget Aunt May. She could provide some emotional support and wisdom as Peter goes through this strange encounter with his parents.
In the 'Peter Parker meets his parents fanfiction', perhaps his parents turn out to be superheroes in their own right. They could team up with Peter to fight a common enemy. This would not only strengthen their family bond but also give Peter a new perspective on his powers and his place in the world.
One possible scenario could be that Loki, in his mischievous ways, decides to play a prank on Peter Parker. Maybe he disguises himself as a normal citizen and lures Peter into a situation where he has to use his spider - sense and quick thinking to figure out Loki's true identity. This could lead to an interesting battle of wits between the two, with Loki's magic against Peter's acrobatic and web - slinging skills.
Maybe Thor. Peter has never seen someone as powerful as Thor with his hammer and god - like abilities. Thor's larger - than - life presence could be really fascinating for Peter.
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.
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.
Well, Loki might see Peter as a new toy to play with. He could mess with Peter's life by using his illusions, like making Peter think his friends and family are in danger. Peter, of course, would try to fight back and save them. This would lead to a chase around the city, with Loki leaving false trails and Peter using his spider - abilities to track him down.
Obviously, Peter Parker is one of the main characters. He's the friendly neighborhood Spider - Man. And then there are the Defenders. This group usually includes Daredevil, Jessica Jones, Luke Cage, and Iron Fist. So all of these characters would be the main ones in this fanfiction.