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.
Often, in Himiko x Kokichi fanfiction, the setting plays a crucial role. If it's set within the context of their original world, the power dynamics and the overarching plot of that world can influence their relationship. Maybe they team up to face a common enemy, and during this process, their feelings for each other gradually develop. Their interactions can be filled with banter, like Kokichi teasing Himiko about her spells not always working as expected.
In 'kokichi snaps fanfiction', it may also be related to the overall theme of the original work where Kokichi is from. If it's from a game, perhaps the events in the game lead him to a point where he can no longer hold back his true self or emotions. The fanfiction could be a way for the author to expand on that moment and show how it impacts the surrounding characters and the story world.