General Feedback From Stealth Games

Stealth games i do like playing and have noticed some key features that seem to do well.

Splinter-cell has been an on going series for many years and therefore must be doing something right. On the bottom left they have a noise meter and also the light meter with the health in the center circle. Quite a neat little compact graphic that works well for the player.

Shooting out the lights also works well for splinter-cell as guards will then investigate which you can then knock out, but the guns would have to be silenced which I'm not just if the pistols in the game will have attachable silencers. If so, it could work ok, just alot more programming to do if implemented.

Another game for stealth although not direct stealth was "The Sabateur", a french resistance game set during WW2. When fully stealth the colours go black and white which is a clear indicticator of when fully stealthed (i think it was also done in some of the other splintercells).

As for other games the endless tiring of "You've been caught, Click x to restart" is just painfully boring and can be a real killer between an alright game and an excellent game.

Hope that helps bring up some issues and i do agree that you need to make more than one option of travel through the levels, as that is what allowed splinter-cell to work, although some levels had just one way through the final bit, but the rest of the level in sections had at least 3 different ways to traverse the NPC's.

Look around corners

IDK if this has been suggested elsewhere, but maybe stealth can repurpose a well polished knife for peeking around objects, which could be replaced with a dental mirror later on.

Basically, when using this, we get a detail shot of the knife/mirror, which can be rotaded a bit to reveal more of the scenery.

It could also be used in some puzzle dealing with lights, or something.

For added realism, maybe after stabbing a Nazi, the knife would be bloody, and auto-cleaned before use (animation in detail shot).

Hiding/Perception

I don't think you should show the degree of hidden-ness of the PC. Seems unrealistic to me.

But if the PC is trying to sneak up on NPCs, he could get an idea based on the behaviors of them.

A smoking guard in a darkened corridor, for example, would extinguish his cigarette, stop pacing, stop muttering, become dead silent, and listen/scan for anything out of the ordinary.
If he feels like he has pinpointed the origin of whatever tipped him off, he might point his flashlight in the general direction, and maybe do a quick sweep around there.
If he has any lingering suspicions, he might disappear around a corner, and wait for anything to pass by him.

A dog might start sniffing, and wagging its tail, cringing with its tail between its legs, or growling. Upon suspicion it might do that hunting-dog-stance (would look comical), bark, whine, or run over/attack.

The PC could hunker down, close his eyes, and receive a boost to his hearing.

He'd pick up fragments of conversation, footsteps, scraping from adjusting rifle- and/or helmet-straps, clatter of rifle-butt against canteen, sounds of reloading/weapons-check, whistling, recital of poetry (or tongue-twisters), unsnapping of buttons (i.e. canteen cover), chugging of water, squeaking doors, squeaking rats, etc...

By turning his head, he could get a sense of direction. If you're not going the surround-sound way, then maybe little directional symbols of sound-waves: ))) ((( or something?

Comparison of previous games' mechanics

Right, this is pretty much just up my alley, since I've liked stealth-based gameplay for all my life. I'll start by comparing different sets of styles and end with a recap, it'll be a pretty big shot again, I'm afraid.

So, when thinking about stealth-mechanics, the main way they're usually handled is a step-based "light-indicator" or a lightbulb or similar object, which shows how well lit your character is. While this is very straightforward and easy to look at, it usually breaks the immersion somewhat. Normal example given of this type of stealth are Thief-games. On the other hand you have the typical 3rd-person game where you show absolutely no sign of how well hidden you are (Witcher 2, Alpha Protocol, Hitman etc.), which can be frustrating at times. The thin line between these styles is pretty hard to make, because I for one can't really figure out how you could combine their elements easily and non-intrusively. Which element suits this game best is pretty much gonna have to be up to you, unless you give us screenshots or action-clips of the mechanics being used in comparisons.

Personally I never played Metal Gear Solid, but I've heard many people praise it's stealth mechanics. Also given that it's a consolegame, I don't trust their ability to judge it widely enough, so I'll leave that in the air, too.

On to the "spot the enemy" part. I've seen games that do peripheral vision animations on enemies (Commando-series), which frankly were necessary for the game to work, but felt stupid. Since when can your commandos "sense" where the enemy is coming and how far he can see. Also, realism is a factor in this, because honestly how many soldiers can only see 15 meters ahead of them, even in daylight. So I'd cross that out, unless you have a helluva lot of soldiers on screen at the same time and you need to be able to see them all to make it through.
Then you have the "I can hear you" model, where the sentry is programmed so that he's whistling, or smoking, or whatever, so your PC can hear (or see the red cigarette butt) when there's a "hidden" guard somewhere not directly lit. These games usually plant guards in spots where there is lighting aplenty, so the player knows lighting == guards. This makes the routes pretty restrictive (more on routing later), the stealth monotonic and frankly, the unseen guards worthy of a reload. Since usually you have to die to the guard or have him alert the base once, before you spot him. Frustrating, annoying and done very many times before.
"Sixth sense" mechanics are a pretty good way of doing it, but don't fit in with this game's concept idea of keeping it real. The Witcher-series for example uses potions which give you the ability to see the vital signs of another being some way away (even through walls). So let's ditch that.
What do we have left? Well, we have ye olde style of actually having the player be shitting their pants about stealth all the time. Think Amnesia: Dark Descent. Very simple game mechanic, very simple tasks to do, ample hiding spots and a lot of things that can go wrong. One of the most terrifying games I've ever had the displeasure of playing, but if you're able to ignore the horror-aspect (the character can't defend himself), it's a grade A++ stealth game. You can carry a torch with you, but it'll give away your position to the baddies. You can go blind, but unless there's some other source of light, you're shit out of luck and also blind.
The way it would work in an isometric sort of system would be so that the player would be given ample time to hide if he'd be spotted. "Halt!" from the guard who sees the light, or whatever. And once you hear the guard is running towards you, you shut down the light and crouch somewhere to hide, wait for him to do his little patrol and maybe even react to it, let him do his small flashlight sweep while you creep behind him and knock him out. Since this is a game with flashlights available, they should definitely be used as "eyesight" markers on the guards in lowlit areas. Forget lit bays or areas, think pitchblack room with wandering guard, looking for the sneaking protagonist. It would also be realistic, since unless the guard shuts his light off, he can't see anything outside it's radius.

Now, once you get used to that idea, here's my big "bang". Light-reactive eyesight. Everyone has it, everyone uses it. Think lowlit rooms when you're about to go to sleep. When you shut down your rooms lightsource, you're blind for a while. But once your eyes get used to it, your room will be pretty much visible (given there's moonlight or any small source of light available). This mechanic should be utilized in such a manner that the PC would have a certain given time in which the rooms visibility would increase in darkness, or not at all if it's pich black. Sort-of-like-bloom-effect but not as tacky and a better alternative. It could be used even in situations like lightning (or a powerful light) showing the whole room and temporarily blinding you because you're used to the darkness etc.

Believe it or not, that's practically never been done before on a bigger scale. And I can't fathom why, it's just a little play on hue and brightness. You'd just have to make it quite close to the real thing, since anything faster or slower would feel weird to the player. Timing would be key. Also, in this model you don't need to know where the guards are, since you can react to them by running away or something. The reaction is key, not the finality of "you've been spotted, try again".

And last, I want to point out a key aspect of sneaky games that's almost always forgotten. Have multiple ways to clear a path or go around it or fool the guards. There's nothing more annoying than having to guess "the right way" of doing it, when in reality you would do something else. Give players a lot of options. A good way of spoiling it has been done in Portal 2. That game had sooo much potential in those large open areas during the mid-section of the game, but what did they have the player do? Search the whole frigging area for that ONE plate that could be used for the portals. There was practically no choice given to the player, only one way to the next door and you had to use it. Such a huge amount of wasted potential.

P.S. About the con-manning. I definitely think the PC should be able to have many different identities (maybe acquired along the way, maybe had them before entering the base) and be able to choose which one to use in a conversation. You'd have to give small hints (and preferably not obvious ones like "use the higher ranking guy on the small soldiers") and maybe observe the guards beforehand. Having the player stop and actually listen to the soldier's banter is a good way to pace the game and make the script-kiddies powergaming through the thing get annoyed. Nothing really increases immersion like a situation where you really have to stop, listen, think, prepare and then try to figure out the right combination so to speak.

Night time on Antarctica?

New Years eve on Antarctica means there is bright daylight all the time on surface of the continent, sun is not setting at all.

It is like midsummer here in Lapland. So all the action must take place in dungeons and tunnels of the military base, if there is dark corners or caves. If the action is happening on surface, then the player could take advantage of the position of the sun in the sky.

Antarctic circle surrounds almost the whole continent: http://www.mapsofworld.com/antarctica/antarctica-political-map.html

And insidethat circle sun is not setting at all in New Years Eve.

So bright daylight is going to shine through all windows to the base. And there is a dangerof getting snowblind outside, UV-beams of sun are reflecting from the snow...