It's time to resurrect this thread and look at what new immersion enhancing mods came out in the meantime. Good times.
-Helgen Reborn [http://skyrim.nexusmods.com/mods/35841//?]: Practically this is just what it says on the tin: The mod not only allows you to assist in the rebuilding of Helgen, it also puts makes you its leader, gives you a bunch of new quests, new armor sets and so much content it could be called a small DLC. It is also a great mod in the sense that it makes you feel the world of Skyrim is actually alive and changes thanks to your actions compared to the lackluster presentation of the vanilla game.
-Run for your Life [http://skyrim.nexusmods.com/mods/23906//?] and When Vampires Attack [http://skyrim.nexusmods.com/mods/28235//?] are two mods from the same author that do the same thing: When Dragons/Vampire Lords attack a settlement, the civilians will RUN AWAY and HIDE instead of throwing themselves at the monsters and getting in your way. Again, works great for the immersion.
-Inconsequential NPCs [http://skyrim.nexusmods.com/mods/36334//?] is a work in progress, and it is a bit of a resource hog, but it can bring a lot of color into your game if you can afford the extra processing power. Simply put, this mod integrates a bunch of NPCs into the game who don1t care about you, don't give you quests, don't want to talk to you and just want to do their business without you pestering them. You know, like real people would.
-Left Hand Rings [http://skyrim.nexusmods.com/mods/21720//?] is a really simple mod that simply adds, well, left handed rings. It is arguably a little silly how you can only wear one ring at a time, and I think upping the limit to two hits the sweet spot where it gives rings much greater utility without making them overpowered by allowing a huge number of rings and amulets like some other mods.
-Dragon Combat Overhaul [http://skyrim.nexusmods.com/mods/32597//?] is, in my opinion, the definitive dragon enhancement. Instead of going the easy route by giving the lizards more health and stronger attacks, the author in this mod actually gave the dragons new abilities and tweaked their AI to make them much deadlier than before. Also, dragons now have some unique abilities and can call for reinforcements, so now meeting a dragon can easily spell doom to even a seasoned Dragonborn. Lovely mod.
-Civil War Overhaul [http://skyrim.nexusmods.com/mods/37216//?] is the one mod I am really looking forward right now, since it actually promises to spice up the civil war quest line without just dumping a bunch of new NPCs into random battlefields. Now you actually have to move with the army of your choice, besiege towns to proceed with the quest-line and, if you do not actively participate, the civil war will move on without you and your side can actually lose it. Can't wait to see the complete mod.