Metal Gear Solid:
if they added a suit that made you effectively invisible while motionless and gave you a gun that allowed you to knock out your enemy with one hit with no repercussions in terms of kill-count, alerting the enemy or any moral dilemmas.
Oh wait, they already did that.
Tranquilliser pistol ruined the MGS series, it destroyed the balance of stealth against combat and undermined any moral dilemma of killing in war. It's also just far too unreal, the reality with tranquilliser is too low a dose and they are merely woozy, too high a dose and they will go into a coma and likely die or suffer severe brain damage and mere drops either way can change that. It's a cop out.
The octo-camo suit too, so overpowered, it removed all diversity to play, it turned the game into a crawling simulator.