To develop Kaguya further, you could show her relationship with other characters in a new light. Let's say she had a friendship or a mentorship with an unknown character in the past. This relationship could have influenced her actions and decisions. It would also add more depth to her character and make her more relatable. Additionally, you could show how she might adapt to the modern ninja world if she were to return, which would require her to change her old - fashioned ways of thinking.
One idea could be exploring Kaguya's past in more detail before she became the mother of Naruto's lineage. Maybe show her life on her home planet and what led her to consume the chakra fruit. Another could be a story where Kaguya somehow returns in Naruto's time and has to face the new world she helped create, with all its different ninja clans and jutsu.
Sasuke would be extremely angry and determined to rescue Naruto. He would immediately start formulating a plan, using all of his skills and knowledge. He might even consider making some dangerous bargains or alliances if it means getting Naruto back.
She could react with surprise at certain parts of the story. For example, if the child says they saw a unicorn at the park, the mother might raise her eyebrows and say, 'Really? A unicorn?' in an excited voice. This reaction encourages the child to continue the story.
In a Giyuu x Shinobu fanfic, the setting and plot can vary widely. Some might be set during the main events of the Demon Slayer universe, with them facing demons together. Others could be more of an alternate universe story, where they meet under different circumstances, like in a modern - day high school AU. The fanfic could also delve into their backstories and how those experiences shape their relationship. For instance, Shinobu's past with her sister could play a role in how she opens up to Giyuu, and vice versa for Giyuu's own traumas.