I would recommend 'Douma's Secret Admirer'. In this fanfiction, the reader is a shy character who secretly admires Douma from afar. It's full of cute and endearing moments as the reader tries to get closer to Douma without being too obvious. It's a really heart - warming read.
Well, the plot could involve Naruto being reincarnated as Douma. He would have to deal with Douma's memories and powers. Naruto, having his own moral code, might struggle with the darker aspects of Douma's character. He could use his ninja skills to fight against the enemies that Douma had in his world, but in a more heroic way than Douma did. Maybe he'd form new alliances with the characters in Douma's world and bring a new perspective to the power dynamics there.
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.