Jujutsu Kaisen has a wide array of characters who were underutilized or underdeveloped, and the case of Mai Zen'in, Maki's twin sister, was the most prominent. While her characterization had a lot of potential to explore, her Cursed Technique, Construction, could have made her a lot more powerful when considering what her Cursed Technique Reversal could be.
Mai's Cursed Technique allows her to create matter out of nothing but is often limited because of her small amount of Cursed Energy.
However, if Jujutsu Kaisen author Gege Akutami had given her a Cursed Technique Reversal, this could have given her the chance to create anti-matter and become strong enough to destroy most of the enemies in the series, at least on paper.
Disclaimer: This article is speculative and reflects the writer's opinion.
Explaining the potential of Mai's Cursed Technique Reversal in Jujutsu Kaisen

Despite being from one of the strongest clans in the series, Mai Zen'in's Cursed Energy output is rather small, thus limiting her own Cursed Technique, Construction.
This ability allows her to create matter out of nothing but takes a major toll on her body, which results in her having to use only one bullet per day so she can keep up, although the potential of a Cursed Technique Reversal is actually quite amazing.
Mai's potential Cursed Technique Reversal, which could be called Deconstruction, would allow her to create anti-matter and become one of the strongest characters in the series because of that concept.
However, as is the case with her initial Cursed Technique, this depends heavily on her Cursed Energy output but is still a lot better than what she is given in the series.
This also shows how the strength of a character cannot just be measured in raw power, which is something that author Gege Akutami has shown from time to time in the manga.
In many ways, it is also meant to depict Mai as an underachiever because of her own shortcomings, although the story should have explored that a lot more.
The wasted potential of Mai's character

Jujutsu Kaisen is known for having a lot of untapped potential but the case of Mai Zen'in is rather tragic because she has several plot points that could have been explored a lot more.
Not only she has a complicated relationship with her sister Maki but she is also torn inside because of her duties as a sorcerer, which is an idea that Akutami should have explored.
This is particularly telling because Maki's slaughter of the Zen'in clan was mostly put under the rub in the manga after it happened, thus making Mai's sacrifice feel a bit pointless in the long run.
Therefore, it would have been interesting to see Mai grow stronger because of the events she went through in the story, but, alas, it wasn't meant to be.
Final thoughts
Mai Zen'in has a lot of untapped potential in Jujutsu Kaisen as a character but she also could have been a lot more as a sorcerer.
That is something that Akutami never explored as a character and is highlighted by the fact her contribution to the story was mostly focused on giving Maki a power-up.
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