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.
Ye Qingmei's last words included three sentences. The first sentence was to tell Fan Xian how to open the box she had left behind. The second sentence was that she hoped Fan Xian would remember that the world she wanted was for everyone to be equal, no matter what kind of person Fan Xian would become in the future. The third sentence was to tell Fan Xian not to ask who his father was because he was only Ye Qingmei's own son. These last words deeply moved Fan Xian, especially the third sentence. It showed that Ye Qingmei's heart was filled with hatred for her father. However, the specific content of the last words was not specified in the search results provided. Therefore, the specific content of Ye Qingmei's last words was still unclear.