One Punch Man's big gimmick is that Saitama, the main character, can defeat his opponents with a single punch. How do you build a game around that? By filling it with a ton of other interesting characters in that same world.
That's the angle Bandai Namco is going with anyway, and they're really embracing it with the whole "Hero Arrival System" motif. The gist is that by "fighting better" and blocking attacks to survive as best you can, Saitama will eventually make his way to the battlefield and let your opponents have it.
It's a clever way to introduce a core concept of the series without sacrificing part of his identity. It'll be on display in the "Free Battle Mode" portion of the game, while focuses on 3v3 combat. In a celebration of characters not named Saitama, Bandai Namco has also revealed that Terrible Tornado, Silverfang, and Atomic Samurai are joining the fray.
One Punch Man: A Hero Nobody Knows has no release date yet, but is confirmed for PC, PS4, and Xbox One. It's been handled by Spike Chunsoft, veteran developer of droves of RPGs and brawlers.