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.
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.
Sure. 'Shinobu's Gentle Touch' is a really sweet giyuu x shinobu fanfiction. It focuses on how Shinobu's kind nature slowly breaks through Giyuu's stoic exterior. 'Giyuu's Hidden Desires' is also interesting as it reveals Giyuu's inner thoughts regarding Shinobu in a very emotional way. And 'In the Moonlight with You' gives a very romantic take on their relationship.
There are fanfictions where Shinobu and Giyuu are paired in a post - demon - slaying world. For example, they might start a new life together, perhaps helping to rebuild the demon - slayer corps in a different way. Shinobu's medical knowledge and Giyuu's combat skills could be combined in interesting ways in these stories.