"The Beauty Catastrophes the World" was a fantasy romance novel. It was about the female protagonist who had changed her body because of a car accident. She was hunted down by her vicious stepmother and was tested to have a useless physique. In a world where martial arts were respected, she was ridiculed and insulted wantonly. However, a useless person could also bully men and women, live a carefree and arrogant life, and harm the world! The female lead was dressed as a man. She was a little rebellious, a little extreme, a little extreme, a little cold-blooded, and a little vengeful. I'm recommending "Beauty Catastrophes the World" to you. I hope you like this fairy's recommendation. Muah ~
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.