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.
Naruto would probably be quite affected. He is someone who has been through a lot of hardships in his own life, so a sad fanfiction about himself might bring back some of those difficult feelings. He could be sad for a while, but then he would likely try to shake it off and remember that it's just a story.