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.
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.
One well - known example is 'The Great Gatsby'. Although not the central focus, there are elements related to water and drowning that add to the overall mood of the novel. The pool where Gatsby is ultimately killed can be seen as a form of drowning, symbolically drowning in his unfulfilled dreams and the corrupt world he was a part of.
The main theme in 'A Red Death' is likely a combination of themes such as the powerlessness against fate, the impact of death on human behavior, and the idea of trying to escape the inevitable. It's a complex and thought-provoking novel in that regard.