You can start by looking on fanfiction websites such as FanFiction.net or Archive of Our Own (AO3). These platforms usually have a large collection of fan - created stories, including those related to Yu - Gi - Oh 5D's characters like Leo and Luna.
One popular one might be those that focus on their adventures in the Satellite. It could show Leo and Luna exploring new areas, facing challenges together, and growing as characters. Maybe they discover some new cards or duel strategies while in the Satellite.
One possible fanfiction could be about Yusei and Jack's early days in the Satellite. Maybe it could explore how their friendship or rivalry began. They might have started as two young duelists trying to make a name for themselves in the rough environment of the Satellite.
One possible fanfiction could be about Leo's transformation into a Dark Signer. It might explore his internal struggle between the dark power and his loyalty to his friends. Maybe he starts to have visions that the dark power shows him, of a world where he rules with great power, but at the cost of losing his true self.
One idea is to have Leo's time as a Dark Signer be a sort of test. Maybe the gods or some higher power in the Yu - Gi - Oh universe are testing him to see if he can resist the allure of ultimate power. In this fanfiction, Leo would constantly be on the verge of succumbing completely to the dark, but there would be glimmers of his true self trying to break through. His duels could be a way for him to either gain more power or lose himself further, depending on how he uses his cards.
One possible plot could be that Naruto, somehow transported to the Yu - Gi - Oh! 5D's universe, meets Luna. Maybe Naruto's ninja skills could be incorporated into the Duel Monsters world. For example, he could use his speed and stealth to help Luna in a dangerous situation, like when she's being threatened by some shadowy organization in the Yu - Gi - Oh! world that wants to steal her powerful cards.
Luna's ability to communicate with Duel Monsters could be combined with Naruto's chakra control. Naruto could transfer his chakra to the monsters Luna communicates with, making them stronger. This way, they could have an edge in battles. Also, Naruto's summoning jutsu could be used to call forth powerful creatures that could work in tandem with Luna's Duel Monsters in a unique combat scenario.
Some popular fanfictions might involve epic adventures where the characters from both series team up. Maybe Naruto and his friends meet Yugi and his gang, and together they face a common enemy. Luna could be the key to unlocking a new power or a hidden mystery. These types of stories often draw in readers who love to see different fictional universes collide and interact in unexpected ways.
A theme of redemption could be interesting. For example, if Jack has made some mistakes in the past that have affected his relationship with Yusei, the fanfiction could be about his journey to make it up to Yusei and regain their trust.
One great fanfiction could be where Yusei and Luna go on an adventure together to find a rare card. It might involve them facing various challenges and relying on each other's skills. Yusei's dueling expertise and Luna's intuition could be combined to overcome obstacles.