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.
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.
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 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.
A common theme is the struggle between Subaru's vampire nature and his feelings for the reader. For example, his vampiric instincts might make him act possessive at times.
A frequent theme is the idea of redemption. They both have their flaws, and in many fanfictions, they try to make up for their past mistakes in relation to each other. There's also the theme of forbidden love, as their relationship is complex and not always accepted within the context of the 'diabolik lovers' world.
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.
There's also 'A Night with Subaru'. This one focuses on a single night where the reader and Subaru are alone in the mansion. The author does an excellent job of building tension and showing Subaru's more vulnerable side. It's a short but very engaging read that really delves into the relationship between the two characters.
Well, let's consider each character. Shu would probably be indifferent at first, but as things progress, he might start to worry about how it could disrupt his lazy lifestyle. Ayato would be envious and might try to cause some trouble for Subaru. Laito would make inappropriate comments and try to get a rise out of them. Reiji would be the one trying to keep everything in order and make sure they follow proper procedures. Kanato would be a bit of a wild card, perhaps being overly possessive or acting out. Yui would be relying on Subaru for support, and Subaru would be ready to fight anyone who threatened his family.