Well, a fanfiction could explore the origin of Atem as a vampire. Maybe he was turned in ancient Egypt and his memories of that time start to resurface. Yugi could be there to support him through the process of dealing with these long - forgotten and often painful memories. This could also tie in with their current relationship and how it changes as Atem uncovers more about his past.
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.
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 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.
One possible storyline could be a time - travel adventure where Yugi accidentally travels back to the era of Atem and has to find his way back with the help of Yami. Another could be a story where they have to team up to defeat a powerful new enemy that is threatening both the modern world and the spirit world. There could also be a more character - centered story where they explore their relationships and how they've changed each other's lives.
A great element is the build - up of their relationship. It should show how they've grown closer over time. For example, through shared adventures and understanding each other's strengths and weaknesses.
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.
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.
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 great 'yugioh yugi x atem fanfiction' story is 'The Bond Beyond the Grave'. It beautifully portrays their connection even after Atem has passed on. Another good one is 'Adventures in the Spirit World' which takes them on a journey in the spirit world together.
Perhaps in the fanfiction, Atem and Yugi have to team up to protect Moka from other threats in the vampire world. Since Yugi is human, he might bring a different perspective and set of skills, while Atem's ancient knowledge and power could be crucial. Moka, on the other hand, could offer her own vampire - related strengths to the group.