What if in his final Battle with Sukuna, Gojo Satoru lost? (Manga ain't there but imagine it somehow ended with Sukuna winning)
Sukuna managed to defeat Gojo, in his final moments thinking about all the people he let down.
(You could adjust the beggining to your liking)
After his death he was floating in the never ending darkness when suddenly he opens his eyes and finds himself being a baby once again.
As he grows up he finds himself in a familiar yet unfamiliar place.
He's still in Japan however there are no cursers or sorcerer but instead there are Hero's and Villains.
On his 4th Birthday, Gojo awakens all the abilities of his past life.
Deciding to not fail again, he decides that he wants to be hero and the best bet is to attend the UA High.
(Plot Twist - Sukuna somehow finds his way to this world and Gojo has to accomplish the thing he failed in his past life. You could do that if you want to add more to the original plotline)