A frequent theme is secrecy. Shinobu's yandere feelings are often something she tries to hide from others, especially from Tanjiro at first. Also, the theme of protection can be seen. Shinobu may see her yandere actions as a way to protect Tanjiro from other threats, real or imagined. This can lead to some interesting plot twists as Tanjiro may not always understand her motives.
One great 'yandere Shinobu x Tanjiro fanfiction' is 'Shinobu's Obsession'. It really delves into Shinobu's yandere tendencies towards Tanjiro. The story shows how Shinobu's feelings start to cross the line from normal admiration to an all - consuming obsession. It's a really interesting read if you like this kind of pairing.
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.
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.