In this fanfiction, Goku vs Jiren is likely to be an epic battle. Goku's power with the Universe's Watch might bring new and unexpected abilities. Jiren, on the other hand, is an extremely powerful opponent. The fight could involve intense energy blasts, high - speed combat, and strategic maneuvers. It might also explore the growth of Goku's character and his newfound power in relation to Jiren's unwavering strength.
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.
A possible fanfiction could focus on a group of Saiyans who are trapped in a dimension where they can only watch the Goku vs Frieza fight on repeat. They start to notice small details in the fight that they hadn't before, like the exact moment Goku's power level spikes or how Frieza is trying to adapt his strategy. This newfound knowledge could then be used by them in some way when they eventually break free from that dimension.
Trunks would also be interested. He has his own experiences with fighting powerful foes and would be eager to see how Goku, a great Saiyan warrior, fares against Frieza. Also, the younger Saiyans like Goten might be interested. They've heard stories about Frieza and would be excited to see their father or other Saiyan heroes in action against such a notorious villain.
In a straight - up power comparison, Goku has the ability to reach extremely high power levels that can destroy planets. But Naruto has the power of the Nine - Tailed Fox and his Sage of Six Paths mode which also gives him immense power. If the fight takes place in Naruto's world, Naruto could use the environment to his advantage, like summoning giant toads or using the terrain. On the other hand, if it's in a more open space like in Goku's battles, Goku could use his flight and long - range attacks more effectively. So it's hard to say who would win definitively.
Then, introduce some shared experiences. Maybe they get trapped in a strange dimension together. In this new and dangerous place, they have to rely on each other's strengths. Goku's physical power and Frieza's cunning could be the keys to their survival. As they go through these experiences, they start to see each other in a new light and their relationship begins to change.
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.