If we swap Star and Marco, then the main antagonists could be those who were previously minor annoyances for the other character. For example, if Marco is now in Star's role, Ludo, who was a main antagonist for Star, could be an even bigger threat as Marco might not have Star's natural magical intuition to deal with him.
If the grandfather is very warm and accepting, Shikaneji would feel relieved and happy. They might open up more about their relationship and their dreams. Nejire could start chatting excitedly about their future together, and Shinsou would relax and join in the conversation more. However, if the grandfather is not so friendly, they would be disappointed and perhaps a bit defensive.
Well, perhaps in this swap AU, their powers are also swapped. Star has Marco's karate skills enhanced to a supernatural level, and Marco can now use Star's wand and spells. Also, the relationships they have with other characters could be reversed. Their friends might react very differently to this new 'them'.
At the moment of the body swap, Aladdin would likely be stunned. He'd be like 'What just happened?' as he looks down at his now - female form. He might feel a bit self - conscious about the new body and the different clothing. Jasmine would be equally shocked. She might be more vocal about her dismay, exclaiming something like 'This is unacceptable!'. But then, being the strong - willed character she is, she'd quickly compose herself and start to assess the situation, thinking about how to get out of this predicament while also trying to understand what Aladdin usually goes through in his daily life.
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.