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.
Tanjiro and Kanao's love is a slow - burning and sweet one. They first meet as comrades in the fight against demons. Tanjiro's kind heart and strong will gradually draw Kanao out of her shell. Kanao, who was initially rather reserved, starts to open up more because of Tanjiro. Their relationship develops as they face numerous challenges together, and their mutual respect and growing affection for each other are heartwarming to watch in the series.
Well, for a Demon Tanjiro fanfic, the atmosphere is key. You need to create a sense of foreboding and mystery. Tanjiro as a demon would have a different aura. You can describe his appearance in detail, like his red eyes that now glow with a more sinister light. The story could revolve around his search for a cure or his revenge against the demons who turned him. And don't forget about the emotional aspect. His relationships with Nezuko and other friends will be completely different. He might be in pain because he doesn't want to harm them but his demon nature might push him to do so.