One possible plot twist could be that Jiren discovers a way to nullify Saitama's strength. Maybe he finds some sort of energy - draining technique that weakens Saitama over time. Another could be that Saitama holds back for some reason, like he wants to see the full extent of Jiren's power, and in doing so, he gets into a real struggle.
A plot twist in this fanfiction could be that there is an outside force that affects the battle. For example, a mysterious entity could appear and give Jiren a sudden power boost that makes him more than a match for Saitama. Or perhaps Saitama loses his memory of his strength during the fight, and has to relearn how to fight, making it a much more even battle. Also, it could be that Jiren has a hidden ability that he has never used before, and it catches Saitama off - guard.
Saitama would win. He has shown incredible strength with just one punch. His power seems limitless, and in a fanfiction scenario, his simplicity and raw power would likely overpower Jiren's more complex abilities.
In a Saitama vs OC fanfiction, a plot twist could be that the OC has the power to make Saitama lose his strength. However, the OC doesn't want to use this power because they admire Saitama and want to test their own strength against him fairly. Another interesting twist could be that the OC is a part of a bigger plan by some mysterious organization. They use the fight against Saitama as a distraction while they carry out their real objective elsewhere. And as the fight progresses, Saitama realizes this and has to decide whether to continue the fight or go after the real threat.
In the fanfiction, the plot could develop such that the power of the universe's watch attracts other villains or powerful beings. They might try to steal the watch from Goku during his battle with Jiren. This would add a new layer of complexity as Goku has to fend off these new threats while still trying to defeat Jiren.
Another plot twist might be that Luffy actually loses the first round of the fight but then finds an inner strength or a new power - up from an unexpected source, like a memory of Roger or a new Devil Fruit ability that he unlocks in a moment of desperation.
Maybe Percy loses his powers temporarily during the battle with Typhon. This would completely change the dynamic of the fight and force him to rely on his wits and the help of his friends in a whole new way. He might have to find a magical item or perform a heroic deed without his powers to turn the tables on Typhon.
One possible plot twist could be that Adam has a hidden weakness that Sun discovers during the fight. Maybe it's related to his past or his weapon. Another could be that Sun is actually being controlled by an outside force and Adam has to decide whether to take advantage of it or help him.
A plot twist could be that during the battle, Ash's Charizard suddenly evolves further, gaining new powers and turning the tide of the battle. Or perhaps Tobias reveals that he has some ulterior motive for the battle other than just winning, like trying to steal Charizard's power.
One common plot twist could be that the reincarnated character has some unique power that interacts with Saitama's strength in an unexpected way. For example, they might have a power that can nullify Saitama's punches for a short time, creating a tense situation where Saitama has to figure out a new way to fight. Another plot twist could be that the character discovers a secret about Saitama's past that even Saitama himself doesn't know. This could lead to a whole new arc in the story, as they try to help Saitama come to terms with this new information. A third plot twist might be that the reincarnated character is actually from a parallel universe where Saitama is a villain, and they have to figure out how to deal with this different version of Saitama.
One possible plot twist could be that Danzo is actually being controlled by an even greater enemy. So, when Naruto is about to defeat him, he realizes that there's a bigger threat behind Danzo. Another could be that Danzo reveals a hidden power that he has been suppressing all along and suddenly becomes much stronger than Naruto expected.
One possible plot twist could be that Naruto discovers Godzilla is being controlled by an evil force. So instead of just fighting Godzilla, he has to find a way to break the control. Another could be that Godzilla is actually a guardian of some sort, and Naruto misinterprets its actions as aggression at first. Then they end up teaming up against a greater threat.