A possible plot is that Harley gets caught in the middle of one of Peter's battles with a villain. Peter has to protect him while also trying to defeat the bad guy. This could lead to Harley learning about Peter's secret identity and then becoming his sidekick of sorts, using his own intelligence to assist Spider - Man in future battles.
Perhaps in the fanfiction, Harley moves to New York and ends up in Peter's school. They initially clash because of their different personalities, but then they discover they both have a passion for helping others. They team up to help the local community, like volunteering at a homeless shelter or cleaning up a polluted area in the city.
A plot could involve two Peters from parallel universes. They initially clash because they have different personalities or ways of doing things. For example, one Peter might be more introverted and scientific, while the other is more outgoing and action - oriented. But as they get to know each other, they start to work together to solve a common problem that threatens both of their universes. This could include fighting a super - villain who has found a way to cross between the universes. Another aspect could be that they discover some hidden secrets about their own identities or powers through their encounter with the other Peter.
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.
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.
Harley is often depicted as a smart and somewhat sassy kid. Peter, on the other hand, is responsible and has a good heart. Their personalities could clash because Harley might be a bit more brash in his ideas and actions, while Peter is more cautious as Spider - Man. But they also complement each other. Peter can teach Harley about the importance of using his intelligence for good in a more selfless way, and Harley can show Peter to be a bit more bold and take more risks in his non - superhero life.
Maybe a plot where Robin and Harley get transported to an alternate dimension. In this new world, they have to rely on each other completely to find a way back home. Along the way, they face strange versions of their friends and foes, and their relationship evolves as they face these challenges together.
One interesting plot could be the Joker and Harley trying to raise their baby in a world that is constantly against them. Maybe they have to protect the baby from other villains who want to use it for their own gains.
Maybe Peter sees Harley having something he desires in the fanfiction, like more popularity or a closer relationship with a certain character.
One idea could be that they team up to create a super - cool invention for Stark Industries. They combine Peter's scientific knowledge from school and Harley's hands - on experience working with Tony. Another could be that they get into a bit of a friendly rivalry over who can impress Tony more with their individual projects.
One interesting storyline could be them teaming up to solve a mystery in their neighborhood that the Avengers overlooked. Maybe it's a small - scale tech - related mystery that Harley's engineering skills and Peter's spider - sense can combine to crack.
A really cool plot could be that Peter gets transported to a different time period in the adventures. He ends up in a medieval - like world where he has to learn the ways of the knights and use his modern - day knowledge, like basic science, to solve problems. He might also have to battle dragons and rescue a princess in this strange new world.