Based on reality and what you can/can't do in RL. I give you a true hardcore mode. I call it the Hardcore 20.
No mining. Only use what you find.
No vehicles. But you can find a bicycle sometimes until it breaks.
No ammo, weapon or outfit crafting
No crafting in your backpack.
No building structures or repairing structures. (unless you find blocks in loot)
Stuctures have at lease 10x hit points. (But only for you)
Weapons/outfits cannot be repaired.
You have to sleep, shower and poop. You are vulnerable when you sleep. Noise can wake you up, but then you are not well rested. Not showering enough attracts zombies. Poop attracts zombies like food does now.
Food and water are far more rare, but food/water consumption is more realistic. Food once every 2-3 days. Water is needed daily. You have meat, water and veggies. This would keep you alive. No fancy magic foods.
Making noise attracts zombies.
Magazines are useful for kindling only.
No magic candy.
No purchasing from vending machines.
All trader locations are abandoned.
Falling from [n#] blocks kills/paralyzes zombies.
You can't have superhero skills. (Jumping 2 meters, falling from more than 2 meters, instant healing etc)
Learn by doing for god sake.
Permadeath
Alcohol kills infection. Not honey. Honey is food.
Zombies can smell your stashed items. Particularly if there is food stored.
Party girl is not attractive and you are not turned on by her at all. What is wrong with you. You have to stop thinking about her like that.
Note that this comes from a single player perspective, not multiplayer. Please destroy/love as your opinion sees fit.