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.
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.
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.
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.
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.
Yugi might be initially afraid of Moka due to her being a vampire, but his kind heart could lead him to try and understand her. Atem, being more fearless, might be more direct in his interactions with Moka, which could either earn her respect or ire. Their interactions would also depend on the setting of the fanfiction, whether it's in a modern - day city or a more fantastical realm.
First, focus on the unique relationship between Atem and Yugi. Since Atem is a vampire, you can play with the idea of forbidden love. Yugi's reactions to Atem's vampiric traits like his need for blood or his enhanced strength can create interesting dynamics. For instance, Yugi might be initially scared but then grows to accept it.
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.