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.
There's also 'Subaru's Redemption in Yui's Love'. This fanfic focuses on how Yui's love starts to heal Subaru's inner wounds. It has some really touching scenes where Subaru slowly opens up to Yui. The descriptions of their interactions are very vivid and it's a great read for fans of the pairing.
There are many great authors out there. One author who stands out is 'RosePetalWriter'. Their works often have great character development for Subaru and Yui, and the plots are very engaging.
A more unique plot could be set in a modern human world where Subaru and Yui are trying to live a normal life. But Subaru's vampire nature keeps causing problems. They have to figure out how to deal with things like his need for blood in a world where vampires are not supposed to exist. This could lead to some really interesting and emotional moments between them as they struggle to be together.
Another interesting plot could be that Yui gets sick, and Subaru is the only one who can take care of her. He has to overcome his own insecurities and fears to be there for her. He might learn more about himself and his feelings for Yui during this time. As he nurses her back to health, their relationship deepens in a more tender and emotional way.
There might be a fanfiction where Yui gets lost in a strange part of the vampire realm and Subaru is the only one who can find her. He battles his own inner demons as well as the external threats to rescue her. This story would show Subaru's protective side towards Yui.
You can start by looking on fanfiction.net. It has a large collection of fan - created stories for various fandoms, including Diabolik Lovers. Just use the search function and type in 'Diabolik Lovers Yui and Subaru' to find relevant stories.
The relationship can be developed through shared experiences. For instance, if they face a common enemy, like a powerful vampire who wants to harm Yui. As they fight together, Subaru realizes how much he cares for Yui and she starts to see a different, more protective side of him.
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.
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'.