I have heard of realism, and I don't know if these elements are in it or not, but I am going to list my idea on a new mutation of realism.
Basically it is so realistic, the only thing that makes it different is graphics, not being able to feel the game in your nerves, and the fact that you still know it is a game.
Here are some features.
Limited handgun ammunition: I am unsure if this feature is already in realism mode, but if not, it should be in Hardcore.
Basically it is what the tin says, you no longer have infinite ammunition on your handguns.
Bleeding: Basically each time you get hit, you bleed for a while, which makes you lose a total of however much HP it should be in a certain period after being hit.
The bleeding effect would wear off and the wound would slightly scab, only enough to stop the bleeding, though you would still not actually see the wound and your character would still look normal.
It would kind of keep a notice on your HUD that you were hit by bleeding, and that you are permenantly more fragile than any survivor who hasn't been hit by bleeding yet.
Basically, falling off even small ledges would cause your slight scabs to rip apart and retrigger the bleeding effect.
Being hit by things such as claws from a Witch, Hunter, or even common infected are what trigger a bleeding effect.
However, it takes several claw strikes from common infected to trigger bleeding, and a Hunter can only trigger bleeding with his claws if he hits you enough, or he claws you in the form of pouncing.
This also goes for Jockies and Smokers.
Internal bleeding: After being crushed by a charger for a total of like 6 seconds, after being released, your death clock has started ticking.
This will make it so you lose HP at a constant rate, and that pain pills are the only things that can slow it.
Eventually, you will die no matter your efforts.
And the pills reducing the bleeding temporarly can be even harder due to another possible feature in Hardcore.
Overdosing: In Hardcore, survivors can only take the ammount of two bottles of pain pills and 4 stimpacks of adrenaline.
If you surpass this limit, your character wil stumble in third person, clutching their chest, then collapsing dead.
This counter would reset after each safehouse.
So if you took the max ammount of medicines, and then you enter the safehouse? You'll be fine if you take another one after the next level loads.
CPR: Defibrillators will not revive someone on a confirmed one try, and they don't work quite the same as in normal mode or even regular realism mode.
First, you have to zap them, which will basically kill their heart. (I have learned a lot about these kinds of things from my mother. Basically, the defibrillator kills their heart when it is beating at an illregular level, and then they use CPR to possibly restart it. This is what they do in hospitals and junk).
Next, you have to go through another action, where your character will lean down and begin performing chest compressions to attempt to restart their heart.
This only has a 20% chance of working, as there is an 80% chance that their heart is done, game over.
Also, if they were hit in the head by a bullet, revival has 0% chance of working and they are dead untill you make it to that safehouse.
Another thing about revival via CPR and defibrillators.
If the body has been there for more than 2 minutes, revival will have 0% chance of working.
First aid: First aid would heal you completely, because I guess this can't be TOO realistic, even though this is intended to be the hardcore version of the already hard realism mode.
However, your character will still limp for about 1 minute and 30 seconds, which then you will resume walking on a normal level.
Death: Death in Hardcore realism would still not be permanent, as you could come back after the safehouse.
However, revival via safehouse can take a very long time, because of how difficult it could be for your squad to make it to the safehouse.
Headshots = instant death: Headshots against anything in Hardcore realism, Tanks, Witches, Chargers, Hunters, Jockies, or even other survivors, equal instant death.
This would be a burden if someone lacks trigger discipline on your team and often shoots comrades, or they are simply trolling and killing their squad on purpose.
However, it can make fighting infected easier in this intensely difficult gamemode, as you could swiftly defeat even tanks with a well aimed headshot to the neural cortex.
So in other words, if you play this gamemode, either have bots who rarely miss and rarely go friendly fire, or have a team of friends which you trust dearly.
I guess this is all my ideas for a Hardcore Realism.
Also like I said, I don't know if anyone has come up with this already, and/or realism already has some of these features.
Please comment and say if you think this would be a very challenging addition to the game, or just be flat out pointless.