The theme of change is quite prevalent. Yui's presence in Naruto's world changes things for him and vice versa. Their combined influence can bring about changes in the village, in the way people think, and in the overall plot of the fanfiction. For example, Yui might introduce new ideas or ways of doing things that Naruto and the others in Konoha haven't considered before.
One common theme is friendship. Since Subaru and Yui are often depicted as friends in the original works, fanfictions tend to explore different aspects of their friendship, like how they support each other through tough times.
One common theme is the power of love. Ayato and Yui often find themselves in difficult situations, but their love for each other gives them the strength to face challenges. Another theme is self - discovery. As they interact, they learn more about themselves.
One common theme is the exploration of their relationship dynamics. Given their personalities, a lot of fanfictions might focus on how they interact, like the shy or hesitant moments between them.
One common theme is the exploration of their contrasting personalities. Yui is often portrayed as gentle and pure, while Loki is more complex and mischievous. Their interactions based on these differences are often explored.
One common theme is friendship turning into love. Often, they start as friends or acquaintances and as the story progresses, their relationship deepens romantically.
One common theme is growth. Yuma and Yui might grow as individuals while being in a relationship. For instance, they could learn to be more confident or more understanding. Also, family can be a theme. Maybe their families have an impact on their relationship, either supporting or opposing it. In some fanfictions, there could be a theme of mystery, like there's a secret about one of them that the other has to uncover as their relationship progresses.
One common theme is love and redemption. Ayato may be initially portrayed as a rather difficult character, but through his relationship with Yui, he might find a path to redemption. Another theme is the exploration of the supernatural elements in their relationship, as they are often part of a supernatural world. And of course, the theme of loyalty and trust also pops up a lot, as they have to learn to trust each other in their complex relationship.
One common theme is the development of their relationship. It could be about how they overcome obstacles to be together, like dealing with family issues in the Diabolik Lovers universe.
Love is a very common theme. Given the nature of the two characters, it could be a complex, often angsty love. There might be a struggle between their different personalities and the situations they are in.
One common theme is the journey of preparing for the baby. This could include things like decorating the nursery or choosing baby names. Another theme might be how the relationship between Ayato and Yui changes during the pregnancy, like becoming more protective of each other.