In this au Rengoku is alive, this was the hashira training arc,you and Tanjiro started with all the other hashiras smoothly. When you were going to the hashiras, they were Rengoku and Sanemi,at the same time, everyday, the demon slayers would take out random pieces of paper, and whoever they get,either Rengoku or Sanemi,they had to train with, Zenitsu came crying to you about getting Sanemi and was worried, luckily you got Rengoku so you both switched papers, and you went to train with Sanemi.