One idea could be a story where Kou and Yui go on a journey to discover Yui's past. Maybe they find an old diary that holds secrets about her family and how she ended up in the vampire world. This could lead to a series of adventures and a deeper bond between them.
First, you can start with a simple idea like their first meeting in a different setting. For example, instead of the usual mansion, they meet in a forest. Just describe the scene, how they look at each other, and what they are thinking. Then build on it from there.
Another common theme is the power dynamic. Ayato has a certain power over Yui in the Diabolik Lovers universe. Fanfics often play with this, sometimes showing Yui finding ways to balance the power or Ayato learning to use his power in a more positive way in their relationship.
Another theme is Yui's struggle to adapt to Kanato's ways. Yui is usually portrayed as a more innocent character, and in the relationship with Kanato, she has to deal with his erratic behavior and the overall dark and dangerous environment of the Diabolik Lovers world. So many fanfics focus on her journey in this regard.
Another theme is redemption. Shu, being a complex character, is often portrayed as someone who might find redemption through his relationship with Yui. It's about how their love could potentially change him for the better. For example, in some fanfictions, he starts to show more compassion towards Yui and others. And also, the theme of sacrifice is quite common. Yui might have to sacrifice her own comfort or safety to be with Shu, and vice versa, as they navigate the dangerous world they live in. This adds a layer of drama and intensity to the stories.
Love and redemption are common themes. Usually, Subaru has a tortured past and Yui's love is seen as a way to redeem him. For example, in many fanfics, Yui's kindness slowly breaks through Subaru's tough exterior.
A common theme is the struggle for independence within the relationship. Yui may strive to maintain her own identity while being in a relationship with Ayato. Also, the idea of redemption can be a theme. Ayato, as a vampire with a sometimes dark nature, may seek redemption through his relationship with Yui. And of course, the theme of forbidden love, given the differences between a vampire and a human, is very common in 'diabolik lovers ayato x yui fanfic'.
Common themes in Diabolik Lovers Ayato x Yui fanfiction include the struggle for control. Ayato wants to control Yui in many ways, but Yui may resist. Love and hate is also a big theme. Their relationship is so complex that it can swing between love and hate easily. And the theme of self - discovery for Yui. As she is in this strange relationship with Ayato, she discovers more about herself and her own strength.
Love and protection are common themes. Yuma often has a protective streak towards Yui. For example, he might shield her from other characters who mean harm. Another theme could be self - discovery. Yui might discover new aspects of herself through her relationship with Yuma.
One possible plot could be Yuma and Yui getting lost in the forest together. Yuma, being protective, tries to find a way out while also showing his softer side to Yui. He might share his past with her, like his experiences growing up, which deepens their connection.