Number one is that it should be modular. If you wanted it active you would have to pick a few things from several choices.
Exposure, Starvation, Sleep Deprivation and Dehydration should be the base but if people do not want the features and want other features that should be an option as well.
Here are some things i think should be options in a hardcoe mode.
-No point to point fast travel. There should be options like mark and recall(only can be used in non combat situations for gameplay reasons), carriages, ships, teleporters and such but not an overmap where you jusst click on the icon and then you go there.
-Expensive fast travel. By expensive, i mean it requires items to use. Like you need food and drink in order to arrive at your destination without being dehydrated, starved and very sleep deprived. If you had food and drink you would arrive less sleep deprived than if you fast traveled without food and drink, but fast traveling would always make that go down. The farther you go the more it drains on your resources or increases your dehyration, starvation and sleep deprivation bars.
-No quest arrows (there should be reasonable alternatives to figuring out where you have to go like NPC directions, notes, clues, etc etc...). Or quest arrows only point towards things on the over world map.
-Increased trap damage/effects.
-Increased disease/poison potency with actually somewhat crippling effects.
-reduced carry weight.
-limb damage?
Feel free to add.