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.
I would recommend 'Whispers of the Heart' for Giyuu x Shinobu fans. This fanfic focuses on their unspoken understanding. It's full of small moments that build up their relationship, like when Shinobu leaves a small flower on Giyuu's doorstep as a sign of her care. Also, 'Shadows and Sunlight' is worth checking out. It has a great plot that involves them having to go on a secret mission together and how that journey changes their relationship from colleagues to something more intimate.
If Luke were kidnapped, in the context of fanfiction, it might be the work of a new, unknown group. This group could be after the knowledge Luke has about the Force. His kidnapping could be a way to force him to reveal secrets. The Jedi Order would be in chaos, and his students, if he has any at that point, would be left without their master. They might try to find him on their own, leading to their own character development and adventures.
One common theme is the exploration of their relationship. Given their different personalities in the original work, fanfictions often focus on how they bridge the gap between them, like how Giyuu's stoicism and Shinobu's more cheerful yet calculating nature interact.
It could be a new villain who has a soft spot for Bubbles. Maybe he's a misunderstood bad guy who just wants some attention. He might see Bubbles as a symbol of purity and kindness that he lacks in his life, so he kidnaps her in a rather desperate attempt to be close to her.
In such a fanfic, perhaps Bubbles would initially be terrified when kidnapped. But as time goes by, she might discover some kindness in her captor. Their relationship could develop gradually. For example, the captor might show her a different side of the world she never knew. And Bubbles, being the sweet and innocent one among the Powerpuff Girls, could start to have feelings for this person who is different from the usual villains she faces. This would create a complex dynamic where she has to balance her new - found feelings with her loyalty to her sisters and the city they protect.