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.
Well, without having read the 'kanae no hoshi light novel', it's hard to say precisely. However, in general, light novels often feature young protagonists. So perhaps this one is about a young person named Kanae. It could be centered around their daily life, which might seem ordinary at first but then takes a turn into something extraordinary. It could include elements like school life, if it's a modern - setting story, or it could be a story about a Kanae who discovers a hidden power or a secret in a more fantastical world. There could be a host of supporting characters that help or hinder Kanae's progress, and various sub - plots that contribute to the overall story.
One idea could be a rengoku fanfic where he trains a new group of young slayers. It could focus on his unique teaching methods and how he instills his values in them.
Tanjiro would be a great character to pair with rengoku. Their different personalities could create an interesting interaction. Tanjiro's kind - hearted nature and rengoku's fiery determination could lead to some great storylines.