Pyre-Z
Saitama starts to use more and more of his full power, and even leaves earth to train. While he is gone, Genos trains with Silver Fang (Bang) along side Sonic to become better fighters. Meanwhile, Dr. Kuseno develops new things for Genos. Then, one day, Saitama returns... but that isn't necessarily a good thing...
Disclaimer: I don not own any of these characters. All i own is the plot i made for this.
Note: This is based of information from Season 1 of the One Punch Man anime and the Manga, with a little bit of info from the webcomic. I will most likely exaggerate powers and abilities, so keep that in mind.