Hunting definitely... I don't know whether it could possibly ruin the game, but maybe I'd like to see PC's having to eat once in a while, or have fatigue work (either to some extent or add something new) like in Daggerfall so you would actually run tired if you didn't sleep once in a while (or cast a spell).
I'm fine with not having things like smithing or tailoring in the game as a PC skill though....!
Having to rest would be cool, I wouldnt mind smithing and such but make it a minigame where the better you do == the better quality stuff you made. Also, what about real darkness outside at night, in Oblivion you could still see a mile away at night outside. Proper darkness might add to exploring/camping for the night strategy.
And maybe some cool cutscene for when you rest, anything but the cheap lil bar they show.