Some fans are devastated. Levi has a huge fan following, and his death in a fanfic can be like losing a real friend for them. They might stop reading the fanfic altogether if they can't bear the thought of him being dead.
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.
Eren would probably be very curious. He's always been eager to learn and understand new things. Meeting kids from another time would be fascinating for him. He might ask them a lot of questions about their world and how it differs from his. Levi, on the other hand, would be more guarded. He'd be suspicious at first, but once he realizes they're not a threat, he might start to warm up and show his more protective side towards the kids.
Tanjiro and Giyuu have a sort of mentor-student relationship. Giyuu offers guidance and support to Tanjiro on his journey, and Tanjiro learns valuable lessons from him. They also develop a sense of camaraderie as they face the challenges of the demon world together.