One idea could be an OC who has a unique deck based on ancient Egyptian mythology that gives them an edge in the Battle City tournament. They could start as an underdog but gradually gain recognition.
One idea could be a female OC who is a powerful duelist from a far - off land. She comes to the Zexal world with a unique deck that has cards no one has ever seen before. This could lead to exciting duels and interactions with the main characters.
An OC could be a young nurse with a mysterious past. Maybe they have a connection to one of the big donors of the hospital. This connection could lead to all sorts of interesting storylines, like conflicts of interest when it comes to patient treatment decisions or uncovering secrets about the hospital's financial dealings.
A plot could involve a mysterious force that disrupts the normal rules of Battle City. Naruto and his friends then have to figure out what's going on while still competing. For example, the battle arenas start to change randomly, and they have to adapt quickly. They might also discover that this force is related to an old enemy who wants to take advantage of the chaos in Battle City.
Perhaps a story where a young, new duelist discovers they have a vampire heritage. They struggle to control their newfound powers while also trying to fit into the competitive dueling world. They might face discrimination from other duelists who fear their vampire nature, but they gradually prove themselves with their unique skills and deck strategies.
One idea could be that the characters from different dimensions team up to fight a common enemy that threatens all dimensions. For example, Yugi and his friends could join forces with counterparts from a dark dimension who have their own unique cards and strategies. This would lead to some really cool duels and character interactions.
One idea could be that the main character, who has traits from both Naruto and Yugioh universes, accidentally creates a portal that brings together characters from both worlds into a shared space. In this new space, a harem starts to form around him as characters from different backgrounds are intrigued by his unique abilities. Another plot could involve a tournament where the rules are a mix of Yugioh duels and Naruto - style battles, and the winner gets to be part of a special harem.
One idea could be exploring the backstories of the dragons in more depth. Maybe there are ancient rivalries among them that were forgotten. Another could be a character getting lost in the spirit realm of the dragons and having to find their way back while learning about the dragons' secrets.
One idea could be a world where the Millennium Items from Yu - Gi - Oh have an effect on Pokemon. For example, the Millennium Puzzle might unlock a hidden power in a Pikachu. Another could be a story where the trainers in the Pokemon world start using Duel Disks from Yu - Gi - Oh to battle in a new, combined form of Pokemon and card battles. Maybe a powerful villain uses the dark powers of the Shadow Realm in Yu - Gi - Oh to control Pokemon for evil purposes.
Maybe a fanfiction where the Queen of Games travels to different dimensions in the Yu - Gi - Oh universe to compete in exclusive dueling tournaments. Along the way, she meets old friends and makes new allies, all while facing various challenges specific to each dimension.
One idea could be that Naruto is transported to the world of Yugioh GX and has to learn the rules of Duel Monsters to survive. He might use his ninja skills in unique ways during duels, like using shadow clones to confuse opponents.