The plot could involve a villain who has found a way to cross between the two worlds. This villain starts causing chaos in both the world of Percy Jackson, like turning demigods against each other, and in Eragon's world, by disrupting the dragon - rider bond. Percy and Eragon, being the main heroes, have to team up. They start by trying to understand each other's powers and the rules of their respective worlds. Along the way, they make new friends and allies from both worlds and face many challenges together until they finally defeat the villain and restore peace in both their worlds.
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.
Well, it could start with a magical accident. In the Percy Jackson universe, a spell gone wrong sends Percy, Annabeth, and Grover into the land of Alagaësia from Eragon. There, they discover that the demigod powers Percy has are seen as a strange and new form of magic. Eragon is initially suspicious of them but realizes they can help in his fight against Galbatorix. They combine their skills, Percy's water - controlling powers and sword - fighting skills, along with Annabeth's strategic mind and Grover's nature - sensing abilities, with Eragon's dragon - riding and magic - wielding skills to take on the dark forces.
One idea could be that Percy and his friends are on a quest and accidentally stumble into Alagaesia, the world of Eragon. There, they meet Eragon and Saphira. The demigods' powers might be seen as strange magic by the people of Alagaesia. Percy could use his water - controlling abilities to help Eragon in a battle against some new threat that is both a danger to the demigods' world and Alagaesia, like a powerful water - based demon. They would have to learn to work together, with Percy teaching Eragon about the modern world's concepts and Eragon showing Percy the ways of magic in his world.
Well, it could be that the Voltron team somehow gets transported to the Percy Jackson universe. Maybe they end up at Camp Half - Blood and have to team up with the demigods to fight a common enemy, like a new type of monster that is a combination of a Galra and a Greek mythological beast. The Voltron lions could be seen as some sort of strange new magical creatures by the demigods.
The plot could involve Percy Jackson's world of Greek gods colliding with the Avengers' universe. Maybe the gods have some sort of issue that only the Avengers' powers can help with, or vice versa. For example, an ancient Greek monster has been unleashed in the modern world that the Avengers are struggling to defeat, and Percy and his friends come in with their knowledge of Greek mythology to assist. It could also be a situation where the Avengers are dealing with a threat that has its roots in Greek magic, and they need Percy's demigod powers to counter it.
Imagine if the gods in the Percy Jackson universe decide to test the mortals in the Kickin It world. They send Percy as a sort of envoy. Percy arrives at the dojo and challenges the students there to a series of tests. At first, the Kickin It gang is angry and confused. But as they engage in these tests, they start to learn about their own hidden potential. Meanwhile, a dark force from the underworld notices this interaction and tries to disrupt it. Percy and the Kickin It gang then have to work together to defeat this dark force. Along the way, they exchange their skills and knowledge, with Percy teaching them about Greek mythology and the Kickin It gang teaching him martial arts moves.
Definitely Percy and Eragon. Percy with his quick - witted and brave nature would have an interesting dynamic with Eragon, who is more solemn and duty - bound. They could learn a lot from each other, like Percy teaching Eragon about having a sense of humor even in tough situations, and Eragon showing Percy the importance of loyalty to a cause.
Annabeth Chase could be an interesting character to focus on. She is highly intelligent and strategic. In a crossover, she could use her knowledge of architecture and battle strategies to help Eragon and his allies. She might study the magic in Alagaesia and find similarities or differences with the powers of the demigods. Her relationship with Percy could also be explored in a new light, especially when they are in a completely different world. How would they support each other in this new and strange environment? And how would she interact with the characters in Eragon's world?
Well, one possible plot could be that the gods in the Percy Jackson universe notice some strange energy disruptions caused by the Infinity Stones in the Avengers' world. Percy and his friends are sent to investigate. They meet the Avengers and at first there are misunderstandings, like the Avengers thinking Percy's powers are from some new form of alien technology. But then they realize they need to work together to stop a common enemy, perhaps a new powerful being that is drawn to the combined power of the demigods and the Infinity Stones.
Perhaps there's a character crossover in a more unique way. A demigod in the Percy Jackson world has a distant relative who was cursed in the Diablo world. This demigod, through some magical accident, gets transported to the Diablo realm. Percy, worried about his friend, follows. In the Diablo world, they have to adapt to the darker and more brutal environment. Percy has to use his skills in a different way, like his water powers to cool down the hellish fires. They meet Diablo - like characters who may not be all bad, and together they try to break the curse and find a way back home.