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.
Dark and spooky settings are common. Like old, dilapidated houses or isolated cemeteries. Another element is the presence of something unknown and menacing, such as a shadowy figure or a strange noise that can't be explained.
Yes, there might be. Fanfiction websites are filled with all kinds of creative works. You can try searching on popular fanfiction platforms like FanFiction.net or Archive of Our Own. Just type in 'Naruto is Skull de Mort' in their search bars and see what comes up.