A really interesting fanfiction I came across was about Yusei's encounters with other Dark Signers before fully becoming one himself. It showed how he was gradually influenced by the dark forces. In some fanfictions, the author might focus on Yusei's new deck as a Dark Signer. It could have cards with darker themes and more powerful but dangerous effects. These fanfictions can offer a different perspective on Yusei's character compared to the original series.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of Yu - Gi - Oh fanfictions, and you can specifically search for ones related to Dark Signer Yusei.
There are many interesting fanfictions on this topic. Some might explore Carly's transformation into a Dark Signer and how it changes her relationships. For example, it could show her relationship with Jack becoming more complex as she is now on the 'dark side'.
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 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 plot could be Carly's initial confusion and fear when she first becomes a Dark Signer. She might not understand what's happening to her and tries to resist the dark power at first.
One possible scenario could be that Yusei gets hurt during a Duel. Maybe his opponent uses a powerful and unexpected card that directly attacks him in the Duel Monsters spirit world, leaving him injured. His friends would then have to come to his rescue, perhaps by finding a way to heal his connection to his monsters or by facing off against the opponent who caused the harm.
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 possible fanfiction could involve Yusei and Akiza going on a journey together outside of the city. They discover a hidden meadow filled with rare flowers. Yusei picks one for Akiza, and she blushes. This simple moment deepens their bond as they realize their feelings for each other are growing stronger. It could be a story full of soft, tender moments and beautiful scenery.
A plot could be that Akiza gets kidnapped by a group who wants to use her psychic powers for evil. Yusei races to save her. When he finally finds her, she's weak but still strong - willed. Yusei's determination to rescue her makes Akiza see him in a whole new way, and they fall in love during the process of her recovery.