If Tanjiro becomes a demon in fanfiction, he might lose his sense of humanity. His relationships with his friends and family would be severely strained. For example, his sister Nezuko, who is also a demon, might be affected in unexpected ways. He could struggle with his newfound bloodlust and the need to consume humans, which goes against his original kind - hearted nature.
Their relationship can be further explored by focusing on their non - verbal communication. Since Nezuko can't speak much as a demon, the looks they exchange, the way Tanjiro gently touches her head to reassure her, these small gestures can be delved into. For example, in a dangerous situation, Tanjiro's glance at Nezuko could convey a whole world of trust and protection.
A great plot idea could be that Nezuko develops a new power that only works when she and Tanjiro are in close proximity. This power could be crucial in fighting a powerful upper - rank demon. For example, she might be able to create a protective shield around them both. Another could be a fanfiction where Tanjiro and Nezuko are invited to a demon slayer gathering in a far - off land. There, they meet other slayers from different regions and learn new techniques. Along the way, they also have to deal with some demons who are trying to infiltrate the gathering.