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.
In yandere x reader fanfics, yanderes usually display intense emotions. Their love for the reader is not the normal kind; it's more of an obsessive love. They may have a dual personality where they can be extremely sweet and loving one moment, but if they feel threatened in their relationship with the reader, they can turn very dangerous and menacing.
One common theme is the exploration of gender roles. Since Tanjiro is male in the original, making him female in fanfic allows for a look at how she would be treated in the demon - slaying world as a female. Another theme is new relationships. For example, how she might interact differently with other characters like Zenitsu or Inosuke as a female.