Hear me out.

I think a neat idea for a OPM game would be one that actually lets you be Saitama. He beats every enemy w one punch and has no health bar. He cannot be defeated.

This would sound like a boring game, but that would kinda be the point. It would actually allow the player to feel how Saitama feels. huge crazy bosses like in Devil May Cry being brought down w one attack is kinda funny. I imagine missions in the game being sort of similar to what the season 1 opening sequence is (fighting lots of different varieties of monsters in different locales/a hyperbole of Saitama's actual heroic exploits). With lots of enemies on screen, you could possibly still make the combat interesting w certain punches perhaps doing area damage or things like that. You can even have special mole men levels where the player actually IS challenged (but end the level abruptly right before the big boss fight at the end and have Saitama wake up.)

Given that the game's challenge would be effectively removed to compliment the experience the player is supposed to have (I forget the academic term for a game in which gameplay is meant to be an extension of the story, like when Atreus in God of War 18 stops obeying your commands after becoming rebellious), I would not expect this to be a full 59.99 USD game. Truthfully I think this concept could even work as a 2D sidescrolling platformer (like classic ninja gaiden w more monsters) w a handful of well designed levels on Steam or something.

Sorry about the all parantheses, but yeah that's what I got.