The 'transformation' trope might be present as well. There could be a possibility that Yugi is turned into a vampire by Atem, either willingly or unwillingly. This would change the nature of their relationship even further, especially in terms of Atem's possessiveness. It could also lead to new conflicts as Yugi adjusts to his new vampiric existence and the implications of being bound to Atem in this new way.
One common trope is the 'forbidden love' aspect. Since Atem is a vampire, their relationship might be seen as taboo in the story's world. Another trope could be the 'damsel in distress' where Yugi is constantly in need of Atem's protection due to his vampire - related enemies. However, this can be subverted where Yugi is actually strong in his own right.
In such fanfictions, the setting often plays an important role. It could be a dark, gothic world where vampires are part of the hidden society. Yugi might initially be unaware of Atem's true nature as a vampire. As the story progresses, he discovers it and has to come to terms with Atem's possessive behavior. This could lead to conflicts between them as Yugi tries to maintain his own identity while also being in a relationship with a possessive vampire.
One common theme is protection. Atem's possessiveness often stems from his desire to protect Yugi. Another theme is the exploration of their deep emotional bond. It's not just about being possessive in a negative way, but more about how their relationship is unique. For example, in many fanfictions, Atem's possessiveness is shown as a sign of his love and loyalty towards Yugi.
One common theme could be the power struggle between Yugi and Atem as vampires. For example, they might compete for territory or influence within the vampire community. Another theme could be their relationship with mortals. They may struggle with the urge to feed on humans while also having some form of emotional connection to them. Also, themes of loyalty and betrayal often show up, like if one of them betrays the other for some personal gain or to protect a mortal they care about.
A really interesting fanfiction idea might be that there is a vampire hunter after Atem, and Yugi has to protect him. This would put Yugi in a tough situation as he has to use his wits and courage to keep Atem safe. They would have to work together, hiding Atem during the day and facing the hunter at night. It would be full of action and suspense.
A common theme is friendship. Their bond is always explored in different ways, like how they rely on each other in tough situations. Another theme is self - discovery. As they interact, both Atem and Yugi often learn more about themselves.
Often, other vampires can be the main antagonists. Maybe there are some elder vampires who don't like Yugi and Atem's new - found power or their different way of being vampires. They could see them as a threat to the traditional vampire ways.
One good story could be 'The Vampire's Bond' where Atem is a vampire and has a complex relationship with Yugi. It explores their connection in a world filled with mystery and danger.
One common theme is friendship. Their bond is a central aspect in many fanfictions. They support each other through tough times, whether it's facing a strong opponent in a duel or dealing with personal issues. Another theme is self - discovery. Yugi, in particular, often goes through a journey of understanding himself better with the help of Yami and Atem. Power and responsibility is also a theme, as they all have powers related to the Duel Monsters and have to use them wisely.
One common theme is friendship. Since they start as partners in duels, their friendship is often explored in depth. For example, how they support each other during tough battles.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, including those about Atem and Yugi in a vampire setting.