If Percy died instead of Bianca, the entire tone and outcome of the fanfiction could be transformed. It might affect how other characters grow and evolve, and potentially introduce new conflicts and resolutions that weren't present before.
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.
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.