Stock plot:
Imagine a post-modern world. Technology controls everything. It's integrated into everything. Skynet, if you will. People go about their daily lives much as they do now, with little a care for the inner workings of the system. But a fringe element wants to take the system down. They believe people rely too heavily on technology and that the technology has moved past the point of being benevolent to culture. The government can't allow that to happen.
The game-play:
The story revolves around two separate threads - Espionage and stealth while working to bring down the government as a character in the fringe element, and squad-based tactical encounters while controlling the government cronies.
The stealth segments would include things like realistic lighting and being able to use shadows to their full effect, as well as being able to create distractions or muffle your movements by way of loud noises. Footsteps and other sounds such as breathing (exemplified by a stamina system altered by the things the player does; a free-running character will be a much heavier stamina user than the spy who hides in shadows, but they will have varying levels of physique that alter how the stamina is used) or misstepping would be taken into account, things such as rubble would be able to be created by the player or used from locations already placed in the game world. Objectives would be reachable by any manner the player could think of.
Need to get to the top floor of a massive building? Blast your way through the front door, find the back door, scale the building itself if you've found the proper items. Or disguise yourself as a worker of the complex, cleverly making your way by all of the guards and other workers in plain sight. Or bribe an NPC to create a distraction for you, even if it might cost said NPC their life. Or hack the power and send the block into disarray, making your way up the tower through the shadows. Or plant an explosive charge on one of the nearby buildings and set it off, causing authorities to come rushing to the scene, meaning you'd need to be a little sneakier because the guards are all on high alert already. Or if you want to just kill the person on the top floor and you don't care about collateral damage, set charges in the basement of the building and take the entire thing down in one fell swoop. Or make your way to the roof of an opposite building and take the target down with a well-placed sniper round. Non-lethal options available for those ethical rebels, with dialogue encounters optional for key targets, allowing you to interrogate them for additional information that might be harder to obtain if you elect for a lethal route.
Story events alter themselves to go along with the player's style. If you go the terrorist route and destroy entire buildings to get at one person, it will have greater repercussions on the world around you. The system will be thrown out of whack by the sheer number of pieces you've removed from the grid, and security might be a bit more disorganized in future missions, but the consequence of which might be that they're far twitchier and much more trigger-happy.
For the government missions, you have indirect control of a squad of government agents, direct control being exerted over whomever is the most important. Missions are a bit more structured than with the rebel faction, collateral damage may have negative repercussions if it goes too far, and the levels encourage you to order your squad about in various ways. You can have them fan out and investigate areas, set up covering sniping positions, evacuate areas you believe house the rebels to minimize collateral damage, or set up a trap if you happen to discover where the objective will take place before the rebels actually show up. Because, oh yeah, the world will act dynamically without direct input from the player.
Obviously there would be some limit to the freedom that the game AI actually has, but it would be entirely within the player's capability to discover things like meeting locations for rebel leaders or the office of a prominent figure in the government and then have the ability to lay a trap designed to take them out while they aren't actually at said location. By the same token, you would be able to miss certain things if you weren't paying enough attention. Targets would be able to get away, dialogue battles would be able to deceive you, the opposing side might actively work toward achieving its goal if you neglect it for too long. The player would be given opportunities to still achieve their own goal, however, even if they failed to stop the opposing side. The consequences would have a greater impact on the game world and whatever organization you're currently playing, though.
And no stupid health-sponge enemies either. One well-placed bullet is enough to take down any human target, and damage would be according to the location you hit. Leg? They fall to the ground, maybe have a difficult time getting back up and are pretty easy to catch. Arm? Well, maybe they could shrug that off if they're hardy. Chest or head? Down for the count, unless they have armor and even then it would cover a realistic portion of the chest and would still likely make them stumble.
The player responds the same way, of course. No regenerating health, no healthpacks, no magic life restoring potions. Bullet wounds to the arms or legs would need to be treated accordingly to stem blood-loss. No gun would be a hitscan weapon, but yes, cover would need to be used liberally whenever you elect to engage in an actual gunfight. You would be able to get Kevlar armor, though, to add some leeway in the difficulty of a gunfight. The enemy AI would attempt to flush you out by flanking in various ways, using high ground, and using things like flash/smoke grenades, hence the heavy encouragement toward being stealthy. Cover would have to be used dynamically. Anything large enough could be turned into cover, but it would also have durability proportionally based on the material of the object. A wood table wouldn't provide much cover against an assault rifle, and a concrete wall might give way after repeated fire. But you would also be able to slip out of a gunfight in multiple ways. Smoke grenade for cover, moving behind cover and slipping off into the shadows, breaking line-of-sight and making your way out of the local area, or, hell, using a grenade to blow out a section of the floor and dropping down to a lower level.
Every enemy target in the game would be able to be taken out in a stealthy way, and if you are spotted, they won't have psychic precognition giving every opponent in a mile-wide radius the instant ability to find exactly where you are. Only an enemy in the same room will know where you are, and they will attempt to warn other nearby enemies of your location but will be able to be cut off. However, gunfire will alert enemies in a fairly wide radius if you're not using a silenced weapon. If you take out one enemy who is in an area that another patrols and move the body away from that area, the patrol will realize something is wrong and will attempt to search you out, which may also cause the area to go into a state of elevated alert.
