Power comparison is a common theme. For example, when Goku crosses into the Marvel universe, fans often like to see how his Saiyan powers stack up against the likes of Thor or Hulk. Another theme is culture clash. When Goku enters a different fictional world like the world of Bleach, his Saiyan values and the way of life in Soul Society are very different, and this contrast is explored in the fanfiction.
In my opinion, it's highly unlikely. Goku has mastered multiple forms and techniques that give him a wide range of powers. Saitama's one-punch style might not be effective against Goku's diverse skills and battle experience.
I don't think there's a clear winner. Both Manga Goku and Manga Saitama have unique and powerful abilities. It could come down to strategy, luck, or even the author's imagination!
One popular mui goku crossover fanfiction is 'Goku in the My Hero Academia World'. It's really interesting as it shows Goku trying to adapt to the quirks - based society. Another one could be 'Goku Meets the Avengers'. Here, Goku's raw power clashes with the high - tech and superpowered heroes of the Avengers, creating some really exciting fight scenes.
Another theme could be the clash of different universes. Amazo is from the DC universe and Saitama is from One Punch Man. Fanfics might play around with how these two different universes' rules and powers interact when these two characters meet. Also, there could be themes related to respect and rivalry. Saitama is often very nonchalant about his power, while Amazo is a complex android with various abilities. How they view each other in the fanfic can create interesting storylines.
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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.
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.
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.