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.
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.
Well, it could be about an OC who uses the powers of the Goro Goro no Mi in a world full of pirates. Maybe they use the lightning - based powers to fight against other powerful foes, like the Marines or other pirates. They might also use it to explore uncharted islands that are filled with dangerous creatures and hidden treasures.
Naruto himself is often a popular character in such fanfiction. Writers might explore how he would adapt to having the Goro Goro no Mi power, or how he would interact with someone who has it.
One interesting concept could be how the Goro Goro no Mi power is used in unique battle scenarios. For example, the user of this Devil Fruit power might combine it with Naruto's signature jutsu in creative ways to defeat powerful enemies.
It could be a new character who has a deep understanding of the Goro Goro no Mi's powers and wants to use Luffy's new - found abilities for their own evil purposes. This character might be a scientist who has studied Devil Fruits extensively and sees Luffy as a prime specimen to experiment on.
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.