Jujutsu Kaisen is a series with a lot of untapped potential and one of its biggest "what ifs" isn't Jogo, who many fans would have wanted to see more of, but rather Mahito. It is not only because of Mahito's sheer strength and how he would have become more powerful, but also due to his ideology and how that would have evolved throughout the series.
When considering the alternate direction of Mahito surviving much longer in Jujutsu Kaisen and escaping from Yuji Itadori and Kenjaku during the Shibuya Incident arc, the character could have learned more from this experience. He was defined by evolution and learning from every event he went through in the story—it is something that could have happened with his ideology as well.
Disclaimer: This article contains spoilers from the manga/anime and reflects the opinions of the writer.
Explaining why Mahito's potential development in Jujutsu Kaisen is the biggest "what if" in the franchise

During the events of the series, Mahito's ideology was defined by his vision that he should prioritize his entertainment and pleasure over anything else, human lives be damned. However, if his defeat at the hands of Yuji Itadori in the Shibuya Incident arc hadn't cost him his life, this would have changed his mentality.
While confronted by the protagonist and his speech of being a cog, this would probably trigger a reaction from Mahito to become stronger and more mature. Before this, his actions were reminiscent of those of a child in the sense that he was taking human lives out of whims and a desire to have fun instead of reaching for something greater.
In that regard, he probably would have craved to become the apex of society in the series, towering above humans, sorcerers, and Curses. In terms of strength, his character had the potential to keep on growing powerful. It could have potentially placed him alongside titans of the stature of Satoru Gojo and Ryomen Sukuna, at least on paper.
The importance of Mahito in the story

There is no denying that Mahito is instrumental in defining the nature of Curses in Jujutsu Kaisen and also being one of the biggest foils to the protagonist, Yuji Itadori. It is because the villain took the lives of several of Yuji's friends, such as Nanami Kento, and slowly developed a rivalry with him, resulting in the latter developing his "cog mentality" regarding how Curses need to be murdered without hesitation.
Mahito is an interesting character in the series because, unlike other villains, he has the ability to grow and develop as the plot progresses, which is something that makes him more dynamic than the likes of Sukuna or Kenjaku. Therefore, this gives the audience a chance to see him put his ideology to the test and adapt to the different challenges he goes through.
Final thoughts
Mahito, not Jogo, is probably the biggest "what if" in Jujutsu Kaisen because of the tremendous potential he has to grow, both in mentality and strength. However, there is an argument to be made that author Gege Akutami knew this and that is why he decided to eventually remove him from the story before he became too difficult to conclude in a satisfying manner.
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