I'd like to preface this post by saying, I'm well aware that my theory doesn't totally hold, but I thought it'd be interesting to bring up.
SO, what exactly do we know about Ryomen Sukuna? According to the manga, he was a real person that became a curse. He had 2 faces and 2 pairs of arms. He's all in all, a villain (and I hate him). But why does he have 2 pairs of arms? Ryomen Sukuna has actually appeared in the Nihonshoki, which is the second oldest book on classical Japanese history, and there's a theory that Ryomen Sukuna was a symbol of twins or brothers (hence the 2 pairs of arms). Exactly which pair of ancient japanese twins/brothers became the spectre known as Ryomen Sukuna is up for debate but a popular theory is that Sukuna was initially the twin brothers, Ousu no Mikoto (the older brother) and Oousu no Mikoto (the younger brother), sons of Emperor Keiko.
According to 'Handbook of Japanese Mythology' written by Michael Ashkenazi, basically what happened was that Ousu no Mikoto had displeased Emperor Keiko, not showing up for meals and taking the women that the Emperor wanted for himself, and so Emperor Keiko, who was angry at Ousu no Mikoto, asked his younger brother, Oousu no Mikoto, to teach his brother some manners. So naturally, Oousu no Mikoto crushed his twin older brother, then tore off his arms and legs, wrapped them in a straw mat and threw them away. ...yeah, Oousu no Mikoto was brutal and Emperor Keiko subsequently became terrified of his son.
Oousu no Mikoto eventually became known as Yamato-Takeru and was granted the Kusanagi, the grass-cutting sword. Oousu no Mikoto had a pretty sick sense of humour, burning a king and his clan in their houses and renaming it 'Yakitsu' which means burning fort, making a pun on yaki, which also means roast. After conquering lands, becoming a hero and emperor after achieving many feats, after his death, when his wives and children assembled to construct his tomb, his spirit transformed into a giant white bird and flew away.
Brutal, a sadistic sense of humour, and an association with the colour white (Sukuna's clothing). My theory is that Ryomen Sukuna in this manga is supposed to be Oousu no Mikoto, or at the very least, the incarnation of his cursed energy, and Itadori Yuji is the reincarnation of Ousu no Mikoto. To me, that explains why Yuji is somewhat able to stop Sukuna from taking over his body but cannot completely restrain him. Whether or not reincarnation is a thing in this manga or not, I think it's an interesting point to bring up, especially since Ryomen Sukuna has actually appeared in classical japanese history text.
Also, Yuji's background is still a mystery, and there is a chance that he may be the child of someone important. My personal baseless theory is that his mother is Tsukumo Yuki and that she might have either placed a heavenly restriction on Yuji or experimented on him, which would explain Yuji's superhuman strength and speed. Perhaps the heavenly restriction or experimentation manifested Ousu no Mikoto's latent cursed energy within Yuji.
