Well, some characters might be amused. For example, Atsushi could be a bit shocked at first, seeing the different interpretations of Dazai in the fanfiction. He might find some of the more out - there scenarios quite strange but also interesting in a way.
Fukuzawa, the president of the Armed Detective Agency, would be a good character to include. His leadership and wisdom can influence the events involving Soukoku. Higuchi, who is loyal to Akutagawa, can also be added. Her interactions with Soukoku can be used to explore the power dynamics within the Port Mafia. And Elise, Mori's ability manifestation, can add an element of mystery and unpredictability to the fanfic.
Popular shin soukoku fanfics usually have a strong emotional core. They make readers feel the connection between the two characters, whether it's loyalty, love, or just a deep friendship. The pacing is also important. It shouldn't be too rushed or too slow. A good fanfic will have a balance, gradually building up the story. Additionally, the fanfic might incorporate elements from the original work's world - building, like the different organizations in Yokohama. This gives a sense of familiarity and authenticity to the readers.
In a Giyuu x Shinobu fanfic, the setting and plot can vary widely. Some might be set during the main events of the Demon Slayer universe, with them facing demons together. Others could be more of an alternate universe story, where they meet under different circumstances, like in a modern - day high school AU. The fanfic could also delve into their backstories and how those experiences shape their relationship. For instance, Shinobu's past with her sister could play a role in how she opens up to Giyuu, and vice versa for Giyuu's own traumas.
I would recommend 'Whispers of the Heart' for Giyuu x Shinobu fans. This fanfic focuses on their unspoken understanding. It's full of small moments that build up their relationship, like when Shinobu leaves a small flower on Giyuu's doorstep as a sign of her care. Also, 'Shadows and Sunlight' is worth checking out. It has a great plot that involves them having to go on a secret mission together and how that journey changes their relationship from colleagues to something more intimate.