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.
One key element is the characters. Tanjiro, as the protagonist, drives the story forward. Another is the demons themselves, with their different powers and ranks. The Twelve Kizuki are important as they are the most powerful demons Tanjiro has to face. Also, the concept of breathing techniques is key.