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.
Campus novels are popular because they are relatable. Many people have gone through school, so they can identify with the characters' experiences, like dealing with exams, making friends, or facing...
It's a story about the adventures of Captain Nemo and his submarine Nautilus. They travel vast distances under the sea, encountering amazing sea creatures and exploring unknown regions.