For example, in hardcoe mode:
1) You would have to eat regularly. The more days you go without eating the more your health goes down. You die if you haven't eaten for 30 days
2) You have to sleep regularly. The more days you go without sleeping the blurrier your screen becomes. If you haven't slept for 7 days you become a non-responsive zombie.
3) A realistic 'carry weight' limit. You cannot carry 6 swords, 4 shields, 5 pieces of armour, 3 axes, 3 warhammers and 67 books all at the same time.
4) No fast travel. Enough said.
5) A realistic punishment if you go to jail. No sleeping in a bed and you're out 2 seconds later. The more serious the crime, the more you serve time. For example, a bounty of 40 means you have to spend 7 ingame days in a cell. A bounty of 1000 means 175 ingame days in a cell. Which you have to serve to continue with your character, no jail break or 'waiting', you need to look at the cold stone walls.
Can't think of anymore right now. But like I said, it should be optional, because I know it's not everyone's cup of tea.