The power level would be insanely high. Naruto already has access to a huge amount of chakra, especially in his later forms like Six Paths Sage Mode. Vegeta, on the other hand, has his Super Saiyan forms which multiply his power exponentially. When fused, they would likely have a power level that could easily rival some of the most powerful beings in both universes.
In a fanfic, Naruto might power up by having a spiritual awakening. He could connect with the spirits of past Hokages in a deeper way, gaining their knowledge and some of their power. This could enhance his existing jutsu and give him new abilities. Vegeta could power up if he were to face a near - death experience against Naruto. This could trigger a hidden potential within him, like a new type of Super Saiyan form that has unique abilities. He could also fuse with another Saiyan (if there were another Saiyan available in the fanfic's scenario) to create an even more powerful being.
Genos could be an unexpected opponent. He might be forced to fight Saitama due to some mind - control or a misunderstanding in the rules of the tournament. It would be really interesting to see how Saitama would handle fighting his own disciple, and how Genos would fare against his master's incredible strength.
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.
A 'bardock vs king vegeta fanfic' could explore the backstories of both characters. Bardock's story as a Saiyan who starts to question the Frieza regime might contrast with King Vegeta's more traditional and perhaps self - serving rule. Their fight could be a symbol of the changing tides within the Saiyan community. It could also introduce new elements to the Dragon Ball lore, like different forms of Saiyan power that are unique to this fictional encounter.
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.
Well, it really depends on how the OC (Original Character) is designed. If the OC has some really unique and powerful abilities that can counter Saitama's strength, like the ability to nullify physical attacks or something really out - of - the - box, then the OC might stand a chance. But Saitama is known for his one - punch knockout power. So, in most cases, Saitama would probably win due to his overwhelming strength that has defeated almost every foe he's faced so far.
A lot of fanfictions play with the idea of Boros having some hidden power or strategy that makes the fight more intense and less one - sided than in the original. For example, he might have a power - up that actually challenges Saitama for a while. Another plot could be a time - travel twist where someone goes back in time to change the outcome of the Saitama vs Boros fight, which then has all kinds of consequences for the One Punch Man universe.