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.
The reason why there were no cars in Frozen and Sleeping Spell was probably to show the power of nature and the importance of the ecological environment. In these two movies, a large number of scenes took place in forests, lakes, mountains and other natural environments. In these environments, cars were not a common mode of transportation but a symbol. In addition, many characters in the movie used horses, mules, and other animals as transportation, which also reflected the importance and respect for nature.