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's character development would take a dark turn. He would start from being a pure - hearted demon slayer to a conflicted being. His internal struggle between his human memories and his new demonic desires would be the main focus. For instance, he might remember his promise to his family but be unable to resist the urge to harm others.