One key element is his human life as Hakuji. His poverty and his attempts to care for his father are important. Another is his love for the girl, which was a major part of his human existence. And of course, his transformation into a demon due to the misdeeds of others towards him.
The 'akaza sad story' might refer to the tragic backstory of Akaza from Demon Slayer. Akaza was once a human with a strong will. He had a family and a normal life, but due to various misfortunes and being turned into a demon against his will, he lost his humanity and was forced to do cruel things. He also had a lot of inner struggles as he still retained some of his human emotions deep down, which made his story a sad one.
Well, Akaza, originally Hakuji. He was a kind - hearted young man at first. He got involved with some bad people who took advantage of his situation. His love for the girl was pure, but they couldn't be together. When he became a demon, he lost some of his human memories but not all. His back story is full of pain and loss, which in a way, shapes his character as a demon. He is a complex character with a past that has both human and demonic elements intertwined.
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.
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.