The Hashiras would likely show concern. Some, like Shinobu, would immediately start looking into possible medical solutions. Giyuu might be more reserved in his worry but would still keep a close eye on Tanjiro. The other demon slayers would also be affected. They look up to Tanjiro, so his sickness would lower the morale of the group. They would try to support each other while waiting for Tanjiro to recover.
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.
Often, in Himiko x Kokichi fanfiction, the setting plays a crucial role. If it's set within the context of their original world, the power dynamics and the overarching plot of that world can influence their relationship. Maybe they team up to face a common enemy, and during this process, their feelings for each other gradually develop. Their interactions can be filled with banter, like Kokichi teasing Himiko about her spells not always working as expected.
In 'kokichi snaps fanfiction', it may also be related to the overall theme of the original work where Kokichi is from. If it's from a game, perhaps the events in the game lead him to a point where he can no longer hold back his true self or emotions. The fanfiction could be a way for the author to expand on that moment and show how it impacts the surrounding characters and the story world.
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.