Well I previously theorized on Sukuna's origin and what Yuji can be.
Searching for stuff online led me to some posts and this book that is apparently used by Gege to model Sukuna on. And in there there was an excerpt about Sukuna and his origin.
It basically went like this
Sukuna used to have a twin. Siamese twin to be precise. But he ate him. It can be just an identical twin as well, like Mai and Maki, but would fit more with Siamese ngl.
A Siamese twin was born to a poor family, who later sold them to a freak show. One day, a cult leader named Mononobe Tengoku purchased several malformed humans, including the Siamese twins from the freak show. Mononobe was fascinated by the twins and kept them in a room without food or water until one of the twins killed and ate the other. Mononobe then attached the arms of the deceased twin to the surviving twin and half of their face. The resulting creature was named Ryomen Sukuna.
That will be Sukuna's origin. He was a nobody, a freak, sold to the circus, and ate his brother. That's how he got the appetite for eating humans in the first place.
Now the origin doesn't have to be the exact same as the above. A more interesting proposal for me is that Sukuna as he is now is the result of the first culling game. The circus=culling game, the cult leader=Kenjaku/Tengen
Sukuna and his twin were in the first culling game and Sukuna was forced to eat his brother for an unknown reason thus far. Probably to win the game or something akin to that.
It's possible that Yuji is a descendant of Sukuna's brother. Gege said in a fanbook Sukuna had no wives or children, but Yuji's entire family looks like Sukuna lol. It's possible they are descendent from him brother's side.
As far as Yuji goes, he could end up being a descendant or another idea I had. And that is that Yuji is Sukuna's twin. That is Yuji is made up of the remnants of Sukuna's twin. Or the other half of Sukuna.
Through Mai and Maki Zenin we see how CT/CE/Jujutsu treats identical twins. And I do not think this was a plot just for the two of them but rather for the wider scope of the series.
Yuji and Sukuna no matter how you look at it look quite similar.
Sukuna comments on Yuji being from that time
Yuji's physicals are on a level of their own. Kinda like the Heavenly restricted people.
Why does Sukuna think Kenjaku does the grossest things?
Cuz Kenjaku used the remnants of Sukuna's own twin to create a vessel that can suppress Sukuna.
Why can Yuji do it?
Why does Yuji have no CT?
Why do Yuji and Sukuna look so much alike?
Why did Sukuna call Yuji a guy from that time?
Why is that gross to Sukuna?
Cuz Yuji is his twin, in essence.
Sukuna - Mai in this context, aka got the cursed technique
Yuji - Maki in this context, aka got the heavenly physicals
Upon Sukuna assimilating and most likely consuming his twin, he evolved to the levels he is now, just like when Maki lost Mai she evolved further.
Yaeh it makes the most sense to me as well. Otherwise don't really see the point if they are all weaker and stuff lol.