Saitama would be shocked at first. He's used to Genos being around, always eager to learn from him. He might stand there in disbelief for a while.
When it comes to Genos' abilities, the Avengers would be intrigued. Genos' ability to fire intense energy blasts could be a great asset in battles. Hawkeye might be thinking about how Genos could provide cover fire or take out tough enemies from a distance. But the Avengers would also be a bit worried about Genos' lack of emotional control at times. In his pursuit of power and revenge, he can be a bit reckless. The Avengers would need to work with him to make sure he doesn't go overboard. Also, Genos' loyalty to Saitama would be something they would need to understand, as it might influence his decisions within the team.
Tornado of Terror would likely be one of the first to try and take on Evil Saitama. She's powerful and confident, but she would soon realize the extent of his power. Mumen Rider, despite being weak, would still stand up to Evil Saitama out of a sense of justice. He would be quickly defeated but his act of courage might inspire other heroes to step up.
Their relationship can be developed by having them face more personal challenges together. For instance, if Genos' family past is explored and Saitama helps him come to terms with it. Or they could have a fight over a difference in opinion, like how to handle a new, extremely powerful villain. After the fight, they realize how much they mean to each other and their bond grows stronger.
One interesting fanfiction storyline could be that Genos discovers a prophecy that Saitama is going to lose his powers. Genos then goes on a mad quest to find a way to prevent this from happening. He visits all kinds of strange places and consults with various powerful beings. Another could be that there is a competition among heroes to see who can train the best new recruit, and Saitama and Genos enter together. They face off against other hero - trainer pairs in various challenges. Also, a story where Genos creates a virtual reality world for Saitama to experience different kinds of battles that he normally wouldn't encounter in the real world, just for Saitama's amusement.
Genos would probably be even more in awe of Saitama. He already idolizes him, but with Saitama being S - Class, Genos might see him as even more untouchable. He could become more protective of Saitama's new status, trying to fend off any detractors or those who might try to take advantage of Saitama.
One way it could be imagined is if a new, extremely powerful villain emerged. This villain might have the ability to nullify Saitama's strength, perhaps by some sort of reality - warping power. In the fanfiction, Saitama could be caught off - guard by this new threat, and despite his best efforts, succumb to the villain's attacks. It would be a very dark turn for the One Punch Man universe.
Definitely Saitama. Genos is strong, but Saitama's strength is on a completely different level. He can take down any opponent with a single punch, and Genos is no exception.
Saitama would win for sure. His power is just unbeatable in the manga.
The villains would have a mixed reaction. Some of the more power - hungry ones might see this as an opportunity to join forces with Saitama and cause even more chaos. However, there would also be villains who are afraid of Saitama's power, even if he's on their side now. They would be worried that he could turn on them at any moment. Overall, Saitama turning evil would completely disrupt the existing power dynamics in the One Punch Man world.
Well, one way to portray their relationship differently is to make it a more competitive one. Genos could constantly be trying to outdo Saitama in small tasks or hero - related activities. On the other hand, it could be a relationship based on mutual admiration from afar. Genos looks up to Saitama but is too shy to get too close. And then there's the possibility of portraying it as a reluctant partnership, where they don't really like each other at first but are forced to work together and gradually grow on each other.