When it comes to stealth though, the Crysis AI can sometimes be a special kind of stupid. I've gone into cloak right in front of them WHILE they're shooting at me/. Instead of keeping shooting for a few seconds or lobbing a grenade in my direction, they stand around for a couple seconds like "Where'd he go?" before fully reacting. And the bad guys KNOW I have a nanosuit that can cloak.
I saw the beginnings of an LP of Crysis 3 on its release, and a CELL soldier at the start shot at the player after sneaking up behind him (not really using stealth against the player; he was just distracted shooting at someone else and got flanked). After the player cloaked, the soldier immediately started spraying the entire area in front of him, just waving his gun around and hoping to hit something.
There's a fine line between making stealth difficult/realistic and impossible. Making those things too realistic could kill the fun factor of the game, which is more important. I could see doing it in a semi-realistic way, where blowing your cover would completely change up how the scenario plays, because all the guards would be on high alert, shift to more defensive positions, no longer patrol alone, etc.
One of the game concepts I've been working on (based on an RP which is in turn inspired by Battle Royale) actually tries to go toward the upper end, with NPCs being able to spot/hear rustling foliage and not necessarily blindly following any sound they hear (so if you throw a rock in a corner to try and draw them away, they may freak out and flee or instinctively shoot toward the noise or go on guard and put themselves in a defensive position). The way it's able to work is that the game also has an RPG-style stat system regarding Perception, as well as hidden stats regarding things like morale (lumped in with fear, so low morale = high fear) so the enemies you're facing aren't going to be trained soldiers or cold criminals.
Another big part of it is that the NPCs in the game can use stealth against YOU, and have access to the full range of abilities that the player has(like sneaking, making noises for a distraction, climbing trees or ducking into crevices to hide, ambushes, etc.). The game idea is more along the lines of Dwarf Fortress or Nethack in that it's explicitly not meant to be fair to the player, and won't cheat to gain an advantage or intentionally fudge actions to make it easier. The RPG stat and skill system governs how the NPCs act, so most enemies will have terrible combat skills and won't exactly pop you in the head from behind a tree at 50 yards before you even hear them coming. But they won't cause a skilled and deadly enemy to miss his first shot just as a warning, or make someone far stronger than you suddenly weaken to keep him from overpowering you in physical combat. The idea is to force the player to focus, be cautious, use their own skill and knowledge to make up for the character's faults, and be a crafty motherfucker.
