One idea could be that Percy Jackson somehow gets transported into the Yu - Gi - Oh world. He might use his demigod powers in the duels, like using his water powers to disrupt a water - themed monster card's attack. Another could be that the characters from Yu - Gi - Oh have to deal with a Greek mythological threat, and they team up with Percy and his friends to solve it.
A cool fanfiction could involve the Millennium Items from Yu - Gi - Oh having some sort of connection to the Greek gods in the Percy Jackson universe. For example, the Millennium Puzzle might be a lost artifact of Athena. Percy and Yugi could then go on a quest together to figure out the true nature of the connection and how to use it to their advantage.
Perhaps in the fanfiction, Percy's sword, Riptide, could be turned into a Yu - Gi - Oh card. When played in a duel, it could have special powers like being able to cut through any monster or trap card. And the Yu - Gi - Oh characters could visit Camp Half - Blood and learn about their demigod powers, which they could then incorporate into their dueling strategies.
One idea could be that a character from Naruto, like Naruto himself, gets transported into the world of Yu - Gi - Oh! VRAINS. He could learn about the Duel Monsters and Link VRAINS technology. Maybe he would start dueling using his ninja skills in a unique way, for example, using his shadow clone jutsu to create multiple copies of himself during a duel to confuse his opponent.
Another concept is a fusion of their powers. Suppose a villain creates a device that combines the power of the tailed beasts from Naruto and the Numbers from Yu - Gi - Oh Zexal. The heroes from both universes have to work together to stop this new threat. Yuma could use his ability to communicate with his monsters and Naruto could use his Sage Mode to sense the new evil power.
The blending can start with a crossover of characters. For instance, Percy could meet Yugi at a special event that bridges the two universes, like a magical tournament that exists in both worlds. Then, they could exchange their skills and knowledge. Percy could show Yugi how to fight physically in a more demigod - like way, and Yugi could teach Percy about the strategies of Yu - Gi - Oh dueling.
One idea could be that characters from both series meet in a new dimension created by a powerful spell card. They then have to team up to defeat a common enemy that threatens both their worlds. For example, Yuya from Arc V and Yusaku from Vrains could combine their dueling skills and strategies.
One idea could be that Percy finds himself magically transported to Skyrim. He could use his water - based powers in the frozen landscapes, like creating ice bridges over rivers or using water currents to his advantage in battles against the various creatures in Skyrim.
A possible fanfiction could involve a crossover where some of the Avengers are sent to Camp Half - Blood. Imagine Tony Stark trying to figure out how the magic at the camp works while also being highly amused by the whole concept of demigods. Steve Rogers could bond with some of the more heroic demigods over their shared sense of duty. Meanwhile, Percy and his friends have to keep an eye on the Avengers to make sure they don't accidentally disrupt the delicate balance of the camp. And then, they all have to face a threat that targets both the Avengers and the demigods, like a group of villains who want to steal the powers of both.
Perhaps there's a plot where the dinosaurs are being used by an evil force. The evil force has found a way to control the dinosaurs and is using them to attack the demigod camps. Percy and his friends then have to figure out how to break the control and save the day. They might find out that the dinosaurs have a connection to an old Greek god or goddess that they didn't know about before.
One idea could be that Percy Jackson discovers a hidden Cybertronian relic. This relic has the power to control water in a way that even Poseidon's trident can't. The Autobots and Decepticons are both after it. Percy has to team up with the Autobots to keep it out of the Decepticons' hands and also figure out how it relates to his world of Greek gods.
In a fanfiction, perhaps the gods in the Percy Jackson universe are aware of Cthulhu as an ancient and powerful being from another realm. They send Percy on a quest to learn more about Cthulhu's weaknesses in case he ever wakes up fully. Percy teams up with some of his demigod friends and they encounter all sorts of Lovecraftian horrors along the way. They soon realize that Cthulhu's power is far greater than anything they've ever faced and they have to rely on their wits and some newfound magic to survive.
In a fanfic, Percy could be a new student at the academy, but with his demigod powers. His water - controlling abilities would be quite a shock to the vampire and monster students. There could be a plot where some of the vampires want to use Percy's powers for their own gain, leading to an exciting adventure of Percy trying to outwit them.