I think that a perfect gimmick for a Superman videogame should be applying the Superman/Clark Kent duality to gameplay, making the player as much of an investigator as a superhero
A challenge in including Superman in video games is that his invincibility (to normal enemies, that is) is too iconic to the character, to the point that it could get old to make him only fight mooks armed with kryptonite or robots. However, I think something that should be indispensable is the superhero/journalist dichotomy in Superman's character, which would not only justify struggles against common enemies in combat but also diversify gameplay.
The idea is that Superman functions as a hero not only in his role as Superman but also in his role as Clark Kent, going on investigations in which a health bar is replaced by a "Cover Bar". Clark will not be truly injured whenever he gets shot, but his cover might get blown. So the player needs to act as if Clark were a human whenever he is in his disguise, or else both the enemies and the Daily Planet will find out he is Superman, and the phase is lost. The most lethal the weapon, the less believable it is that a mild-mannered journalist can survive getting hit by it.
For example, let's say that Superman finds out that there is a clue to an evil plan inside a factory. But he can't break in as Superman, or else he would alert the criminals he needs to investigate and interrogate. Therefore, the player must enter as Clark Kent in an investigation for the Daily Planet and evade attacks by the enemies. Each time Clark gets "injured," the believability of his lies decreases ("It just grazed me!", "That was a close one, I almost got shot!", "Good thing I'm wearing a bulletproof vest!") until his cover bar depletes and the player needs to restart.
Therefore, the player would use phone booths spread around Metropolis to transition between the two personas as needed for different missions and goals.
If the player needs to overtly use superpowers, such as in map navigation and boss battles, they switch to Superman; if they need to sneak in to investigate an area in which Superman wouldn't be allowed inside without alarm, such as a crime scene or an evil lair, they switch to Clark Kent. Every time the player is in Clark Kent mode, they get access to Lois Lane and Jimmy Olsen as helpful companion NPCs. For example, if there is kryptonite in a region, the player switches to Clark in a nearby phone booth and gets it away with their help, expanding the map in a way they could not as Superman.
This mechanic could also lead to tense situations in the narrative. Superman couldn't just get into LexCorp, but as Clark Kent, he could interrogate Lex Luthor for clues in missions in which the player would need to be extra careful to not blow his cover.
The gameplay mechanics change from action-adventure as Superman to stealth detective as Clark. To keep Clark's sections from being too monotonous, though, Clark should still get to use his powers: super hearing and x-ray vision would be allowed all the time, but the others could only be used in secret; if the player misses the timing, the Cover Bar decreases. For example, Clark needs to use his laser vision to open a coffin with a clue inside, but there are enemies nearby: he must distract them first before getting the right timing to use his powers.