Most poll options are self-explanatory, except perhaps these:
Fast travel requiring food supplies and money for the trip, and random encounters and other consequences
Fast travel is so easy right now, fleshing it out a bit would probably be a welcome addition even for casual players. But what I'm proposing is an option that you can turn on and off, so no worries of it interfering with your gameplay.
First, for long distances it should feel like a trip and not like teleportation, so you can arrive tired, with a message telling you how your trip went and how long it lasted.
Second I think that selecting an itinerary i.e. which roads you take would be interesting, and would allow showing your traveling progress on the map like in the Indiana Jones movies. There are great roleplaying opportunities here, since a wanted thief will probably want to avoid frequented roads, legion forts, etc.
Third, random encounters: if the game tracks your itinerary you can come upon a blocked road for many reasons, and you have to fight, talk or reason your way through (or maybe walk the wilderness for a bit to go around the obstacle) to resume fast travel.
Fourth, supplies: packing food and money before leaving on a trip just makes sense, you don't stop existing during the travel...
Guilds requiring loyalty and time spent on bureaucracy as you rise in rank, making leading several guild an art in itself.
I always found it stupid that you can become master of all guilds so easily, and that rising in rank implies no responsibilities. You're supposed to be the Archmage, but you have no duties and nobody cares if you don't show up for months.
I think that as you assume positions of power, politics should come in play: jealous rivals try to point out your failures and search your past to prove you're corrupted, too much involvement with another guild or frequent absences raises suspicions about your loyalty and involvement. Of course any suspicion that you're involved in an enemy guild or in activities abhorrent to the guild should get you in big trouble and force you to abdicate your position if they are proved (or if people believe in lies about you...). On the other hand you should be able to play the same tricks on your superiors... false accusations and set-ups included.
Being present a certain amount of time should also be a requirement. I'm not actually suggesting the player should idle in the Archmage's office looking at papers, but maybe it could work like the "serve your time" feature in jail. Working in an office could also affect your stats . The main point here is that you can't be everywhere at the same time, and if two guilds expect you both to be present at some point, trying to juggle the two will get you in hot water. Maybe this should be abstracted by you actually filling an hour grid, so the parts where you're doing administrative work in various guilds are abstracted out as long as they don't conflict: you just play regularly in the free time that's left.
Of course to prevent this from becoming too heavy the player could find all sorts of excuses to go on extended vacations and "research trips". I see these features as a very interesting way of livening up guilds after their quest lines are finished: you could be busy trying not to lose your office, then regaining it by regaining the confidence of your na?ve guild members and then exacting vengeance on those who ousted you... lots of fun possible here, and it would be easier than with Oblivion to create such quests, thanks to the quest conditions system Beth has created for Skyrim.
Diseases that are serious business: contagious, require specific treatment, initial symptoms leave you guessing at what you caught
I find Oblivion diseases annoying because they damage your stats but don't really add any gameplay... just run to the chapel and they're all gone. On the other hand imagine for a moment that in Skyrim the game doesn't tell you right away that you caught a disease, and then you notice some ugly rash on your skin or your screen blurs briefly from a bad headache... "uh oh... what did I catch in that dungeon? Or in that... uh... pleasure house?" Then it's the run to different doctors and alchemists who of course give you contradictory advice and try various medicines on you... you just hope they're not crooks.
Now that's what I'd call gameplay. Of course a balancing effort is needed, since if you keep catching horrible diseases the whole game will consist in visiting doctors. On the other hand curing a particularly tenacious ones would be an interesting ongoing quest (especially if you gradually realize it's called Porphyric Hemophilia...), and you could develop immunities to some stuff you've already had.
Check out the amazing Oblivion mod http://www.tesnexus.com/downloads/file.php?id=25835 which pretty much nailed it, even though it doesn't implement all the things I'd like to see.
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Comment away and vote! If you come up with other interesting immersion options, I'll probably add some to the poll above. No promises though.