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.
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.
A plot idea could be that Yui gets sick with a strange illness that only Shu can cure because of his unique vampire abilities. This forces Shu to confront his feelings for Yui as he tries to save her. Or perhaps they find an ancient prophecy that involves them and they must journey together to fulfill it, facing many obstacles along the way and growing closer as a result.
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 plot idea might involve a situation where the kiss is witnessed by one of the other brothers. This could lead to jealousy and conflict among them. For example, if Laito sees the kiss, he might try to drive a wedge between Ayato and Yui out of his own twisted sense of fun and envy. Or perhaps the kiss unlocks some hidden memories for Yui that are related to her past and her connection to the Sakamaki family.
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.
If Yui and Subaru were pregnant in the fanfiction, it could start a whole new arc in the Diabolik Lovers story. The family might be divided on how to handle this situation. Some might see the baby as a new hope or a way to gain more power. Subaru would have to grow up quickly and face his responsibilities. Yui would have to deal with the physical and emotional changes of pregnancy in a world full of danger. Their love for each other could be tested as they navigate through this difficult and unexpected situation.
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.
Perhaps Yui discovers a power within herself that is connected to Ayato's past. As she tries to understand this power and its implications, she and Ayato are drawn closer together. They might have to search for answers in old vampire lore, and during this journey, their feelings for each other grow stronger. For example, they could find that Yui's power was once a key part of Ayato's family's history, and this revelation brings them closer on an emotional and spiritual level.