Novel GOJO: A Sorcerer In The Soul Society
Short version: Gojo dies and gets bleached.
Serious version: After an epic fight against Sukuna at full power. Gojo sacrifices himself to protect his students, Yuji and Megumi.
What he didn't expect was that he would wake up in the afterlife.
NB: This is set 55 years before the start of canon. For the fan of Bleach, 110 BC, 55 BC, and 20 BC are very important milestones. I choose 55 because it would help me make an interesting story.