The future is never set in stone, and it's proven when Gojo gets a letter in the mail. Or, author didn't see much of JJK watching the future fics so she made one herself.

Tanjiro, Inosuke, and Zenitsu were rescued by Shinobu and Tomioka after their mission on Mt. Natagumo. While the three swordsmen are taking their sleep, a sudden noise and a blinding light sent them to a mysterious room alongside with the Hashiras and their swordsmiths. What would they react to see Tanjiro's previous life and how he became one of the greatest Demon Slayers in the present.