An Open world Superman Game that takes place in Metropolis, similar to the PS4 Spiderman and the Arkham series, this is not an origin story, you play as a Superman in his prime. He has his most famous powers from the start: Super Strength, Super Speed, Flight, Heat Vision, X-Ray Vision, Frost Breath, and Invulnerability.
The main enemy would be Brainiac and he has an army of robots with advanced alien weaponry capable of damaging Superman, would be the in game reason as to why he can be damaged and killed if he receives enough damage, the gameplay would be like a beat' em up, sure Superman is not a particularily flashy fighter but you would be able to make some crazy combos by mixing abilities like his heat vision and speed, his Speed could work similar to the Venom Strike from Miles Morales (There one where he lunges fast at an enemy and sends him flying), or be used as a dash to avoid attacks as well and there can be finisher moves like Superman freezing an enemy and then shattering it with a punch, or just punching them into space.
Aside from Brainiac there would be other Superman foes like say Bizarro, Lobo, Lex Luthor, Metallo who would serve as bosses and like in Arkham other villains like the Toyman and Conduit would be teased but not encountered in game, there could be a Doomsday easter egg where you can find underground a large metallic door and you hear something pounding on it and growling but you cannot open it nor use your xray vision to look past, maybe as the game progresses the pounding gets louder and the door starts to show some damage.
Like the Arkham series you can also fight random criminals, but the difference here is that Superman is completely immune to guns, knives and even rockets, now I know what you are thinking "Where the fun in that?" the challenge here would come on either minimizing collateral damage, for example you arrive to stop a bank robbery but the robbers have taken hostages and will execute them if you don't stop them before they do, Now Superman is of course not a stealth character, so the gameplay in this sections would instead revolve around how creatively can you use Superman's powers to stop the robbers, like using his speed to take out the robber with the hostages like that scene from Hancock:
Of course not all Human enemies will have hostages, some would have bombs, others will have neither and be easy pickings, other could have procured weapons from the fallen Brainiac drones or be using Kryptonite bullets. Human enemies would need different finisher moves as you can't have Superman punch a random robber into the sun
And lastly you would be able to unlock multiple customs and similar to PS4 Spiderman they would give you different abilities, for example the Black Suit from the dead of Superman would maybe give you regenerating health in combat but nerf your other powers, the Man of Steel suit will allow you to perform a special finisher where Superman breaks an enemies neck

and of course a Nicolas Cage suit and we get Nicolas Cage to redub all the dialogue with his voice....
Yeah its never gonna happen but one can dream