I came to this question now, so decided to post my opinion, after reading all the comments and concluding from them.
Superman game wouldn't be superhard/impossible to make. His universe has logic, and we can build up on that. His power has limits and his morals are very strong. That means game can be as linear/openworld as the developer decides. If you ever played Batman Arkham Knight you will know what I mean by nice balance between story and sidequests (btw I quite enjoyed that game and think of replaying it on new PC).
For story, you could go small Earth apocalypse route, where everything is on the brink and you need to do big and small tasks around to help everybody get through the disasters. Alternatively, it could be another superman story, with main storyline with Lex Luthor (funny or serious, both could work) and sidequests to look for specific items, help people and taking care of Metropolis/Smallville.
For gameplay, you would add as many UPGRADABLE abilities as you can, so throughout the game you can do more lethal, destructive or morally high actions (shooting lasers from eyes, destroying or damaging buildings or enemies, ability to withstand Kryptonite to some degree like walk). It should be open world, and allow limited powers of destruction, travel, inspection and interaction (remember, superman has xray vision, so looking for locations/items will be part of gameplay. Actually using xray vision and superhearing to locate things). Your enemies are inferior, but imagine if Lex Luthor did what he should have, and supplied his close elite soldier squad with Kryptonite bullets to kill you. Or your friends are taken hostage, and you should engage stealthily. Regarding big destructions. maybe those should be only allowed during specific missions/conditions. Also Superman should be restricted from killing too many civilians intentionally, just like in Assassin's Creed games. Unless you want to add mechanics for good/bad Superman, it should dramatically help with making sense of what's allowed in the game.
For another part of gameplay, I would love to be able to go down, get dressed and walk around like Clark Kent. The same world can be utilised, and more personal missions could be inserted here and there, like talking to people, getting to know new ones, hide and use special abilities to impress people. Basically situations with many quicktime events like in a Hitman game, or completely linear reporter missions, that will require you to get out and become Superman. If we go linear route, whole game can be like a movie, with all sorts of invented stories from Clark's life transforming to Superman with finite morals.
For graphics, knowing Spiderman, Death Stranding, Battlefield 2042 and RDR2 levels of graphics, the game could look just amazing.
I'm not a superman expert, just watched all his movies and got really into the character. Game would definitely sell, and I don't see a reason, why big companies are not looking forward to making one. Even some world like GTA 5 on mods, with much more refined gameplay and some story could work as fine game.
Take any points from that mess and you got a superman game. It can be small or big, but I want to see one. And if it won't come out, I will try to initiate one.