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.
I think 'Carrie' by Stephen King is among the best slasher novels. It tells the story of a girl with telekinetic powers who is bullied at school and then takes revenge in a very bloody way. The exploration of high school bullying and the resulting rage is really well - done. Also, 'American Psycho' by Bret Easton Ellis is notable. It gives a disturbing look into the mind of a wealthy and deranged serial killer in the 1980s Wall Street scene.
One of the main themes is adventure. Since rengoku is often involved in battles and dangerous situations, the fanfics may take the reader along on these adventures. They might face demons together and their relationship develops through these shared experiences. Also, loyalty is a big theme. Rengoku is a very loyal character, and this loyalty extends to his relationship with the reader in the fanfics.