I guess this is now an AU????
Anyway maybe Genos gets sent to the past or an alternate universe where Saitama turns out to be more like Mumen Rider (Can’t get powers no matter how hard he trains) but he still wants to be a hero despite this.
SO when Genos shows up Saitama is astonished by how strong he is but is confused as to why someone like Genos wants to be around him. Like, here’s this murder machine cyborg who can decimated a mountain with a single blast of fire, but instead chooses to tag along with a loser like him?
Genos meanwhile has extreme faith in Saitama despite his lack of powers and firmly believes that he can achieve the same strength as his alternate self in time, or at the least become a great hero without them.
….and then shipping happens.