In a 'kokushibo meets tanjiro fanfiction', one possibility is that Kokushibo senses something different about Tanjiro right away. Maybe it's the fact that Tanjiro has a unique scent or an energy that Kokushibo has never encountered before. Tanjiro, being Tanjiro, would be ready to face Kokushibo head - on, but also be open to communication if there's a chance. The story could explore their different views on the world of demons and slayers, and how they might come to understand each other better through their encounter.
For Tanjiro, he could become more understanding of the complex nature of demons. Meeting Kokushibo, a powerful and complex demon, might make him realize that not all demons are simply evil. Kokushibo, on the other hand, could start to have some doubts about his loyalty to Muzan. Tanjiro's tenacity and belief in the good of all might start to chip away at Kokushibo's cold - hearted exterior.
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.
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.
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.
Another theme is mentorship. Shinobu, being more experienced, often acts as a mentor to Tanjiro. She imparts her knowledge about demons and fighting techniques. This is seen in quite a few fanfictions where Tanjiro looks up to Shinobu for guidance.
Well, through the Dance of the Fire God, Tanjiro's character develops in multiple ways. Firstly, his sense of purpose becomes even more defined. He realizes the importance of the dance not just for himself but for the whole world he's trying to protect from demons. His compassion grows as well. When using the dance, he might see the pain of the demons more clearly, which in turn makes him more empathetic. His leadership skills start to emerge too. Other demon slayers might look up to him when he showcases the power of the dance, and he has to take on more responsibilities. Also, his determination gets a huge boost. Each time he performs the dance, he is more determined to end the reign of demons.