One possible theme could be alternate universes. Fans might write fanfictions where Banri and Koko are in different settings or situations than in the original story. This could range from them being in a different time period, to having different occupations. Another common theme could be dealing with past traumas. Since both characters have some elements of their past that haunt them, fanfictions may focus on how they face and overcome these traumas together or separately within the context of their relationship.
Love and relationship development are likely common themes. Given the nature of Banri and Koko's relationship in the original work, fanfictions may explore different aspects of their love story, such as how they overcome obstacles.
Growth and self - discovery could also be themes. Banri's journey of rediscovering himself might be further explored in fanfiction. Koko's growth as a character in relation to Banri could be another aspect. For example, how she matures in handling their relationship. Also, themes of friendship within the context of their relationship might be present. Maybe how their friends influence or react to their relationship in the fan - created stories.