the only way i could see this properly pulled off would be have the player create a character, hero or villain, who starts out at the bottom of the barrel, and as you develop into someone stronger you slowly go up the ranks of hero or villain hood.
You could start as a guy who is simply working out and then learn martial arts, gets some robot parts, and boom you are now an a class hero. The other option would be to start as some weak villain, and then slowly as you cause more destruction you would become stronger and if you are not human, you would evolve into something stronger.
opm is a deconstruction of the usual stronk manga protagonist. without him, we are quite literally leaved with generic the shonen manga, electric bogaloo, revenge of cliches.
that could be better...except if we are to believe saitamas flashbacks, he didnt fight anybody before crabman. and by then he was already pretty much overpowered already...
How about a OPM game in which instead of fighting his enemies, he has to make the fight more and more interesting for him so he doesn't get bored, you get a bonus grade if you force the enemies to go into final forms before you defeat them
I want Metal Gear / One Punch Man crossover game now
Big Boss tries giving Saitama CQC lessons
Gets punched so hard he's knocked out of the timeline
Then if you save and load up Revengeance, he flies right into Armstrong
Armstrong explodes into a shower of muscles and nanomachines
A confused Raiden looks on as Big Boss stands and dusts himself off
Then he turns to Raiden and says,
"Kept you waiting, huh?"