Perhaps a story where Percy and Natasha are both sent back in time to ancient Greece. There, they have to work together to change a certain event in history. Along the way, Percy gets to show Natasha his world of demigods and she gets to use her spy skills in a very different setting.
Another idea is that Percy gets thrown into the Marvel universe and Natasha is the first one to find him. She initially thinks he's some kind of new super - powered threat but soon realizes he's from a different realm. They then have to figure out how to get Percy back home while also dealing with the various threats that are attracted to Percy's presence in the Marvel universe.
A possible plot could be that the gods in the Percy Jackson universe and the Avengers in the Marvel universe are somehow connected through a dark magic that is starting to disrupt both worlds. Percy and Natasha are chosen as representatives from their respective sides to investigate. They travel to different dimensions together, facing strange creatures and uncovering secrets about the connection between their worlds.
One interesting plot could be Percy trying to balance his demigod powers with the skills he might inherit from Natasha. For example, his water - based powers could interact in unique ways with Natasha's spy and combat training. Maybe he uses his control over water to create distractions during a mission, like making a sudden fog while he sneaks around using Natasha's stealth techniques.
The character dynamics. Percy is often portrayed as brave, a bit reckless at times, but with a strong sense of loyalty. Natasha is a complex character, a highly skilled spy with a troubled past. Their interactions can range from initial mistrust to a deep friendship or more, depending on the story. For example, Percy's powers can be used in creative ways to help Natasha in her missions, and Natasha's skills can protect Percy in a world he doesn't fully understand.
One common theme is the clash of their different worlds. Percy is from a world of gods and monsters, while Natasha is from a world of spies and international intrigue. Fanfictions often explore how they bridge this gap.
One great fanfic could be where Percy and Natasha team up on a mission. Maybe Percy uses his water - controlling powers to help Natasha infiltrate a base near a large body of water. It would be an interesting crossover of their different skills.
One idea could be that Natasha is on a mission in the wizarding world. Maybe she's tracking down a dark wizard who has some sort of connection to a powerful and dangerous Muggle weapon. Harry Potter could help her navigate the magical aspects of the case, like dealing with spells and magical creatures.
Another idea is that Peter gets lost in a dangerous part of the city and Natasha has to find him. Along the way, she discovers his unique abilities and how they can be used in different ways. She then starts training him to be more in control of his powers while also protecting him from the threats in that area.
One idea could be a story where Tony and Natasha have to go undercover as a couple on a mission. They start to develop real feelings during the process. For example, they might have to attend a high - society event and deal with all the pretenses while also trying to uncover a hidden enemy. Another plot could be set in a post - apocalyptic world where they are two of the few survivors and have to rely on each other completely to stay alive and rebuild society. Or perhaps a story where they both get trapped in a virtual reality world created by an enemy and have to find a way out together while also facing their own inner demons.
One idea could be Natasha going back in time to prevent a major event that led to the loss of many lives. For example, she could stop a certain villain from getting a powerful weapon in the past. This would create a whole new timeline and she would have to deal with the consequences of changing history.