Well, in the fanfiction, the moment when Cosmic Garou tries to counter Saitama's attacks with his own new - found cosmic powers is really exciting. He has evolved to a whole new level, and seeing how his powers match up against Saitama's is a big part of the thrill. Maybe he creates some sort of cosmic energy blast that Saitama has to dodge or block in an unexpected way.
The battle between Saitama and Cosmic Garou in the fanfiction would have a catastrophic effect on the environment. First, Saitama's initial attacks might send out concussive forces that uproot trees and scatter them like matchsticks. Cosmic Garou's response could involve creating some sort of cosmic vortex that sucks in everything around it, from small pebbles to large boulders. As the fight progresses, the area around them would become a desolate wasteland, with fires raging from the heat generated by their attacks and the air filled with dust and debris, making it difficult for any living thing to survive in the vicinity.
For a good character interaction between Garou and Saitama, start with their fundamental differences. Saitama is a hero who is bored with the lack of challenge, while Garou hates heroes. Have them start with a fight, but make it a comical one where Saitama's nonchalance annoys Garou to no end. This can lead to some great dialogue exchanges where Garou tries to taunt Saitama into taking him seriously, but Saitama just doesn't care.
One possible plot could be that the Child Emperor, with his high intelligence and various inventions, is initially intrigued by Garou's self - proclaimed 'hero - hunting' ideology. They might have a series of battles where the Child Emperor uses his technological gadgets to counter Garou's martial arts skills. Eventually, they could find some common ground in their views on the nature of heroes and villains, leading to an unexpected alliance or at least a mutual understanding.
It's really hard to say. The Child Emperor has his amazing technology and intelligence on his side. He can create all sorts of gadgets to attack and defend. But Garou is a really tough fighter with great martial arts skills and the ability to quickly adapt in battle. If the fight is in an open area, Garou might be able to use his speed and agility to dodge the Child Emperor's attacks and get close enough to land a powerful blow. However, if the Child Emperor has time to set up his defenses and traps, he could potentially wear Garou down. So it could go either way depending on the circumstances of the fight.
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.
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.