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.
There was this girl who always had her headphones on. One day, she sat by the window. She took a deep breath in, and the music in her headphones seemed to amplify. It was a beautiful melody. As she breathed out, she closed her eyes. In her mind, she was in a meadow, the music creating the scene. With each inhale, she felt more alive, and with each exhale, she drifted further into the story the music was telling.
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.