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.
In Days Gone, the main character Deacon's relationship with his wife Sarah is at the core of the love story. His journey is driven by his love for her. He is constantly searching for her in the post - apocalyptic world, which shows his deep emotional connection and love.