I'm in on some quests should have a urgency behind them, not all but some. The "Someone lies ill and is in danger of dieing" variety should have a timer to them and encourage you to do those quickly. But the "Find me this artifact" variety usually don't need a timer. Just the ones that would feel odd if you could hang around forever.
Also for the main quest there should be alternate paths at least, like if an important person in the quest line dies it tries to continue another way. If that person only assigned you missions they could still be given to you by someone else, if the person was essential the path could change entirely. Like that you don't need an alternative path for every single person in the main quest line that could potentially die but only for the ones really essential to it.
In Oblivion for example, if Jauffre would die early on his position could simply go to a successor who acts on instructions left behind OR you directly act on said instructions. If Martin however died it would require a new quest lines but there could still be potential to finishing the main quest an alternate way.
Also for the main quest there should be alternate paths at least, like if an important person in the quest line dies it tries to continue another way. If that person only assigned you missions they could still be given to you by someone else, if the person was essential the path could change entirely. Like that you don't need an alternative path for every single person in the main quest line that could potentially die but only for the ones really essential to it.
In Oblivion for example, if Jauffre would die early on his position could simply go to a successor who acts on instructions left behind OR you directly act on said instructions. If Martin however died it would require a new quest lines but there could still be potential to finishing the main quest an alternate way.
Seems to me that if you want to add urgency to a quest then give it a due date.
I liked having invincible NPC's, to me it gave things a sense of realism at times (considering that most of the people who were npc's wouldn't commit suicide) and it never really took anything away. In Fact, Hanging out with the Germaine brothers was pretty awesome because I didn't have to constantly worry if they were going to die during some pissant fight.
I like the idea of having to sleep, eat and drink.
I really like the idea of diseases progressing in effect. Some diseases can really knock you on your ass for a while and it'd be nice to see that in game.
No fast travel other than in-game solutions like boats and some form of taxi's (ie:morrowinds stilt striders) would add to the realism.
No quest marker in the hud makes sense but sometimes marking your menu map would be more realistic than no markings at all so I'm going with no hud marker as the answer.
Limb crippling and even bleeding wounds would be realistic but obviously I don't think that a VATS system should be implimented.
It doesn't make much sense to carry more than a couple thousand gold pieces without it starting to get heavy so a bank feature would be cool. For big purchases you'd want to clear a little room in the pack.
I don't know if this would make sense to be a part of a "hardcoe mode" feature but it sure would be more realistic if you had to have certain skill requirements to rank up for most of the guilds. especially considering you could become the arch mage with very little real magic ability. In addition, it would be more realistic if you actually had MORE responsibilities rather than say...none, once you got to a top position in any guild or faction.
Oh, and in a hardcoe mode I think it would be better to have days that are twice the length as normal mode to make up for the amount of things that need to be done and in addition to that, when you go to the menu screen it slows time down by a lot rather than pausing the game. This way it'll give you a sense of urgency while switching weapons or find that perfect spell or potion to use. Plus, that makes it so you have to find a nice safe place to read the books rather than reading an entire book in between strikes of your blade while fighting a skeleton in a dungeon with no lighting but the torch you brought but aren't using. That would also make time pass, slowly, while you create potions and repair your items so you can't make a thousand potions in one day.
Maybe clothes could wear down over a long time too giving an incentive to buy different outfits, in fact that might give an opportunity to add a new "realism" bar called "cold". Skyrim has many mountains and it could be interesting to have to dress the part a little. If your clothes are worn down and if you are in snowy environments long enough your COLD meter goes up eventually compromising your agility, endurance, willpower and intelligence or something until you find some shelter or new/thicker clothes or maybe even until you sit by a campfire or use a fire spell.