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.
Tsukuji, Shinobu, and Masayoshi were the three main characters in the tennis prince. Their pairing was often referred to as "Shinobu Masayoshi" or "Shinobu Masayoshi" for short.
A possible tennis prince's fanboy is Atobe's gift. The story was about the emotional entanglement between Atobe and Masayuki, as well as Atobe's cruelty and ruthlessness towards Masayuki during the competition.
In this article, the three Sugawara Fuji, Shinobu, and Shinki all had strong strength and deep emotional backgrounds. Their pairing was often controversial. In the storyline, Atobe's cruelty and ruthlessness towards Makoto in the game made people feel heartbroken, and Makoto's feelings towards Atobe were also very complicated.
Well, some popular rengoku x reader fanfics can be found on Wattpad as well. Look for ones with high ratings and good reviews. They usually offer different storylines, from adventure - filled ones where rengoku and the reader go on quests together, to more romantic and heart - warming stories about their relationship.