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.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
The battle of Yoriichi vs Kokushibo can be found in the advanced stages of the Demon Slayer manga. It's an intense and highly anticipated confrontation. The build-up and action are both masterfully depicted.