You could write a fanfic where the 'ashes ashes we all fall down' is part of a children's game that has a dark and mysterious origin. The main character discovers that the game was once used to sacrifice people in an ancient ritual. Or, make it a time - travel story. A person travels back in time and realizes that the phrase was a prophecy in a long - forgotten era, and they must prevent the events that lead to the 'fall down'.
For an 'ashes ashes we all fall down fanfic', you might consider making it a psychological thriller. The 'ashes' could symbolize the main character's past traumas that are slowly consuming them, and the 'fall down' is their mental breakdown. It could be about a detective who is investigating a series of murders where the killer leaves behind ashes at the crime scene, and the detective slowly realizes that they are also being targeted by the same force that is causing everyone to 'fall down'.