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.
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.
First, study their characters. Know their personalities, backstories, and how they interact in the original story. For example, Shinobu is often calm but has a hidden fierceness, and Giyuu is a bit of a loner. Then, come up with an interesting plot. It could be about them going on a joint mission where they face a new type of demon. Keep in mind their relationship dynamics as you write.
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.