There was a pretty cool mod for Oblivion that actually makes fatigue restoration worth your time. Don't pay attention to your fatigue and you'll notice that as it drains your movement speed slows and if it hits zero you collapse.
Instead of implementing a hunger or hydration bar, I'd personally like Hardcore mode to simplify the HUD and provide a more involving approach. With Hunger and Thirst being invisible stats and directly tied to your Health and Fatigue. When you're fatigued you'd get thirstier and hungrier and when you're thirsty and hungry your health and fatigue don't regenerate as quickly.
With my ideal Hardcore Mode you'd have no health or fatigue bar, you'd instead have idle animations and tells for different conditions:
Hunger
When you're hungry you'd hear your character's stomach and occasionally hear them groan or grumble when it gets more intense.
Thirst
When you're thirsty you'd hear your character smacking their lips, struggling to swallow or trying to clear their throat.
Fatigue
When you're fatigued your character wouldn't run as fast and as you get even more fatigued begin breathing heavily and eventually rasping for breath.
Health
When your health is low your character might initially just groan every now and then in pain, with more serious injuries resulting in laboured breathing and coughing up blood.
To stop yourself just dropping in the middle of combat you'd have another hidden stat: Adrenaline. Your adrenaline would build up quickly in combat or dangerous situations and slowly decrease when in a calm environment. It's basically a failsafe to make sure your hunger, thirst and fatigue don't drop to zero in the middle of combat and get you slaughtered.
Restoring your hunger and thirst would be a matter of simply carrying food and drink in your inventory. You could either eat/drink through a mapped key/inventory or your character would eat automatically when idle.
All of this being said, it's probably too convoluted out there. I'd also like to see a body temperature stat that affected performance as well, so you'd have to choose your armour according to the climate. It'd also give Frost and Fire resistance another use.
