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.
The themes usually include unspoken love. Since both characters are not very vocal about their feelings in the original story, fanfictions often explore the idea of them having deep, unspoken emotions for each other. Also, there's the theme of support. They support each other in battles against demons, and this aspect is often elaborated on in fanfictions, like how Shinobu's medical knowledge helps Giyuu when he's injured.
How about 'Giyuu x Shinobu: The Jealousy Factor'? This title clearly shows the relationship between the two characters and the key aspect of jealousy. It gives a hint to the readers about what the fanfic might be about, which is the jealousy that could exist in their relationship.
One great 'shinobu and giyuu fanfic' is 'The Unspoken Bond' which delves deep into their complex relationship. It shows how they gradually come to understand and support each other in the face of various challenges in the Demon Slayer world.
There's also 'Shinobu and Giyuu: A New Chapter'. This fanfic focuses on a post - main - story scenario where they team up for a special mission. It's really interesting as it shows their growth from their initial somewhat tense relationship to a more harmonious partnership.
The plot could start with Shinobu noticing Giyuu getting a lot of attention from a new recruit. She starts to feel jealous as she realizes she has feelings for Giyuu. Giyuu, on the other hand, is oblivious to Shinobu's feelings and the new recruit's admiration, which causes more tension.