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.
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.
One popular storyline could be them going on a training journey together. Genos, being the eager learner, and Saitama, with his overpowered strength, could teach Genos new techniques in unique ways. Another could be them facing a new, extremely powerful villain that challenges even Saitama, and Genos has to step up in unexpected ways to assist. A third one might be a story where they get transported to a different dimension and have to find their way back while dealing with the strange new world and its inhabitants.
They have a master-student dynamic. Saitama is the master, and Genos is the eager student looking to learn from him.
Saitama is undoubtedly stronger. He's the one-punch hero who can defeat any opponent with a single punch, while Genos, despite his enhancements, still can't match Saitama's power.
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.
Another plot could be Genos getting caught in a cross - fire of a battle between two extremely powerful cosmic entities. His body is severely damaged and despite Saitama's attempts to get him help, his internal systems fail one by one. His death could then lead to Saitama realizing the true fragility of life around him and make him more protective of the remaining heroes.
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.
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.
They have a master-student kind of relationship. Saitama is the master and Genos is his disciple, constantly seeking to learn from him.
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.