One common theme is family loyalty. Given the strong bond within the Kamado family in the original story, this is often explored further in fanfiction. Another theme could be redemption, especially when it comes to Nezuko's situation as a demon. Writers might also focus on the theme of survival, as the family has to deal with the constant threat of demons.
Demon Kamado family fanfiction can be really exciting. It could focus on the untold stories of the Kamado family. Maybe there are stories about their ancestors and how certain family traits were passed down. Some fanfictions might also create alternate universes where the family has different powers or face different types of demons. It gives fans a chance to expand on the original story and get creative with the characters and their relationships.
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.
One common theme is the exploration of their relationship. Given their different personalities in the original work, fanfictions often focus on how they bridge the gap between them, like how Giyuu's stoicism and Shinobu's more cheerful yet calculating nature interact.