A possible plot could be that there is a prophecy in both worlds that only a combined force of demigods and elves can stop a great evil. So, characters from both universes are brought together. They have to overcome their differences in culture and abilities. The elves might find the demigods' fighting style too rough, while the demigods might think the elves are too secretive. But in the end, they work together and use their unique powers to defeat the evil.
Another idea is that Sophie Foster from KotLC travels to the Percy Jackson universe. She could be mistaken for a demigod at first. She would have to learn about the Greek gods and monsters. Her telepathic abilities would be really useful in the Percy Jackson world. For example, she could communicate with some of the monsters to find out their plans. And Percy and his friends would help her understand this new and dangerous world.
Fitz and Grover could also have a unique interaction. Grover is very in - touch with nature and has his own kind of "sixth sense" for danger. Fitz, being an elf, also has a connection to nature in his own world. They could bond over their love for nature. Grover could introduce Fitz to some of the Greek nature spirits, and Fitz could show Grover some of the beautiful and magical places in the elven world. They might also have some comical moments when they try to understand each other's languages and customs.
Another possibility is that Percy somehow gets transported to Eragon's world. He has to adapt to the new environment and figure out how his demigod powers work there. Eragon could initially be suspicious of Percy but then they realize they can help each other. Percy could teach Eragon about water magic, which is new in Eragon's world, and Eragon could show Percy how to ride a dragon, which would be an amazing experience for Percy.
A third idea is a role - reversal type of story. The demigods from Percy Jackson end up in the world of Fire Emblem Awakening. Percy could use his water - based powers in a unique way in the battles of Fire Emblem. Annabeth could use her strategic mind to help the Fire Emblem characters plan their battles better, and they all work together to defend the kingdoms in Fire Emblem Awakening from an invasion.
One idea could be a crossover with the Harry Potter universe. Percy could visit Hogwarts and have to deal with both magical and mythological threats. Maybe he'd team up with Harry to defeat a common enemy that combines dark magic and ancient Greek monsters.
One idea could be that Percy and his friends get transported into an anime world like Naruto's. Percy could use his water powers in the deserts of Suna and have epic battles against the sand ninjas. They might also learn new skills from the Naruto characters, like chakra control, and in return, teach them about Greek mythology.
A crossover with Harry Potter could be cool too. Percy could visit Hogwarts and participate in a Triwizard Tournament - like event. He'd be the outsider with his unique demigod powers, and there could be a lot of fun interactions between him and the Hogwarts students, like Hermione being intrigued by his abilities.
One idea could be that Percy and his friends from Camp Half - Blood are somehow transported into the Doom universe. Maybe they have to fight the demons there using their demigod powers and the weapons they find in the Doom world. For example, Percy could use his water powers to fight the fire - based demons.
Well, perhaps she's on a quest. The Avengers mistake her for a threat at first. She's looking for a magical artifact that has the potential to either save or destroy both the demigod and superhero worlds. Along the way, she teams up with some of the younger Avengers like Spider - Man or Shuri. They don't trust each other fully at the start, but as they face various challenges together, they start to respect and rely on one another. And she's constantly in touch with her demigod family, getting advice from them about how to handle the superhero situation.
Another idea is that Peter discovers the existence of demigods while investigating a series of strange disappearances in Central Park. He stumbles upon Percy who is on a quest. They initially have some misunderstandings as they come from such different worlds, but then they figure out that they can help each other. Percy helps Peter understand the magical side of things, and Peter helps Percy with his knowledge of the city's layout and technology.
Well, one idea for this crossover could be that Savitar somehow ends up in the world of Percy Jackson. Maybe he's chasing after some new kind of speed - the speed of a demigod? And Percy has to figure out how to deal with this strange, super - fast being who doesn't seem to follow the normal rules of the Greek myth world.