29(Demon Slayer)

An Au where Tanjiro does not become Demon King Tanjiro after the final battle, he just passes outand things follow canon until his supposed death at 25 when the blood Muzan gave him acts and turns him into a demon who kills everyone precious to him.

Uzui is one of the last surviving people together with one of his wives but soon DK Tanjiro finds them and kills them too, Uzui expectd that to be the end and hoped that Tanjiro would forgive himself if he ever gained his conscience back.

But for some unknown reason, Uzui wakes once again and finds himself in the past on the same day as Sanemi was presented as the new hashira.

He almost thought he was under an illusion but not even a demon couldmake one so realistic so he decided that he would change things and save his friends and family.

Of course that Uzui tells Kagaya about what happened as he wouldneed the master's help to have more then one hashira go on a few missions(Kanae's, Giyuu meets he Kamados, Mugen train, Red Light District, etc) and up everyone's training(both hashiras and normal slayers).

Some of the few things he does is befriend Giyuu and Sanemi who had been his closest friends especially the first one(he even treated Giyuu as his 4th wife bit without the kisses in the mouth, sex, etc) before trying to intervein in a few events he was told about and even deciding to approach some of the senseis(Jigoro as an example) to watch their students and maybe help train them(I like the idea of Zenitsu becoming Uzui's tsuguko).

How anything else would go like how Uzui prevents the deaths, if Uzui was the hashira with Giyuu who met the Kamados or itwas another, if Kanae while not dying stops being an hashira due to injuries, if Uzui beats some sense into Shinjuro(and maybe even Kagaya...god knows how hard it was to Kiriya and the other two daughters to have lost their parents and both elder sisters in the same day), what ships would be there(canon or otherwise), etc is up to the writer.