Yes, in some fanfictions, Tanjiro is depicted as the reincarnation of Yoriichi. This concept adds an interesting layer to the story as Yoriichi was an extremely powerful and legendary figure in the original lore. The idea allows for new storylines where Tanjiro might inherit some of Yoriichi's skills or have a deeper connection to the past events related to Yoriichi.
One implication is that Tanjiro may have access to Yoriichi's combat skills. Yoriichi was a master swordsman, so Tanjiro could potentially inherit his swordsmanship techniques, making him even more formidable in battles against demons.
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.
Tanjiro might use his sense of smell to detect hidden threats. Since his sense of smell is super - enhanced, he could sniff out any demons or traps in the way of saving Rengoku. For example, if there are demons disguised as allies, Tanjiro would be able to tell by their scent.
Tanjiro's fate in the manga is that he ultimately passed away. This event was a significant part of the story's development and had a profound impact on the other characters and the overall plot.