Akaza's death in the manga was a consequence of his inability to overcome the powerful teamwork and determination of the heroes. Tanjiro's special abilities and the support of his companions ultimately led to Akaza's downfall.
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.
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.