MC dies in his world and gets reborn in the world of Demon Slayer as Sukuna from Jujutsu Kaisen
You should have the MC become a demon and write a story from the point of view of the Demons.
Obviously keep his original personality and abilities from Jujutsu Kaisen since they are badass
So this idea isn't your 'hero' MC story, its a story about a ruthless MC who does whatever he wants.
Have him fight the main characters in Demon Slayers.