WARNING: The following contains spoilers for Jujutsu Kaisen Chapter #160, "Colony," by Gege Akutami, John Werry and Snir Aharon, available in English from Viz Media.
Jujutsu Kaisen's current primary antagonists, Kenjaku and Sukuna, have remained shrouded in mystery since their initial introductions. Despite very little being known about them, they are among the most interesting characters in the series. They are both directly involved with the series' protagonist, as Yuji Itadori is Sukuna's vessel and Kenjaku's son. It is believed Kanjaku and Sukuna were both alive during the Heian period in Japanese history, making them over 1,000 years old.
There have been many hints already pertaining to both villains' pasts. Fans have a lot of questions, and the best way to answer them is with a flashback arc. Jujutsu Kaisen has already had two of these: the Cursed Child and Star Plasma Vessel arcs, both of which fleshed out several major characters. Here's why Sukuna and Kenjaku deserve the same treatment.
Previously in Jujutsu Kaisen, Satoru Gojo told Yuji that Ryomen Sukuna is known as a fierce imaginary god, but in fact he was a human who actually existed. Sukuna lived as a human over 1,000 years ago, during the golden age of jujutsu. In Japanese history, this was during the Heian period. He is the undisputed King of Curses and is so powerful that jujutsu sorcerers were unable to fully destroy his body after his death. Hence, Sukuna was able to traverse the ages as various cursed objects via his fingers, which were sealed in grave wax, constantly becoming more powerful.
Meanwhile, Kenjaku's main goal in Jujutsu Kaisen is partly to revive the golden age of jujutsu, implying he also lived during that era. However, it's unclear whether that's the time period he originated from or if he's perhaps even older. It would be interesting to see what Kenjaku and Sukuna's lives were like a thousand years ago, and why exactly this period is known as the golden age of jujutsu.
The fact that Sukuna was once a human brings a lot of matters into question. A flashback arc could explore how and why he became a cursed spirit. As he is now, Sukuna the King of Curses is evil incarnate, but it's possible he wasn't always like that. If that's the case, seeing what made him become so carelessly cruel would be fascinating.
Kenjaku's cursed technique allows him to take control of people's bodies by transplanting his brain into a target. Noritoshi Kamo was known as history's evilest sorcerer over 100 years ago, but it was actually Kenjaku controlling his body. He inhumanely experimented on both humans and curses as he tried to fuse them together. Choso and his brothers in Jujutsu Kaisen resulted from this. Yuji may have been an experiment himself, considering Kenjaku took control of a woman's body to create him with Jin Itadori.
Sukuna and Kenjaku have the potential to be Jujutsu Kaisen's most compelling characters. There are many intriguing mysteries surrounding them that would make for an entertaining flashback arc. Yuji being both Kenjaku's son and Sukuna's vessel makes the possibility for such a storyline all the more enticing. It's no wonder fans of the manga aren't just hoping for one -- they're expecting it.