A big, big change is getting mods that effect NPC interaction. That's what got me to play Skyrim again. It's a subtle change that translate to a significant one in game. Here's some ones I can recommend:
The People of Skyrim:
Really nice mod that changes a lot of aspects about both the game world and NPC behavior. Expands a TON on the world by adding new villages and NPCs to make Skyrim look less like an abandoned land where everyone migrated south for the winter.
https://www.nexusmods.com/skyrim/mods/87687/Immersive Citizens:
What I thought People of Skyrim was at first. This mod is all about expanding vanilla NPCs' behavior and adding to the sense of realism, which breaks up the feel that all NPCs in Skyrim are weird robots. It adds new schedules for a ton of NPCs, so they're not just wandering around town all day.
https://www.nexusmods.com/skyrim/mods/65013/Interesting NPCs:
Adds a bunch of NPCs around the world that are all fully voice acted and unique from each other and from the base game characters, but more or less fit into the Skyrim world (they all should be lore friendly). Many are followers.
https://www.nexusmods.com/skyrim/mods/8429/To Your Face:
A brand new mod that gets rid of that awkward dialogue that happens when you're running through a town or sitting in a town marketplace and people talk to the back of your head over each other because they all think the Dragonborn gives a shit about what they tell them every time they come into town. It's an SKSE plugin, so you'll need to get SKSE and drop the plugin into the SKSE plugins folder. There's tutorials online that explain how each type of mod should be installed, by the way. More or less you should just use Mod Organizer and learn where mods are supposed to be.
https://www.nexusmods.com/skyrim/mods/87635There's also some mods that introduce new way quests can be gained, meaning new quests are added. I haven't used many but here's some I know were pretty popular.
Missives:
Adds signposts in several towns and cities that offer local jobs, similarly to the monster quests from the Witcher 3.
https://www.nexusmods.com/skyrim/mods/85815Legacy of the Dragonborn:
Another one I always heard great things about but never used myself. Adds a playerhome that you can fill with the legendary things you find along both the vanilla questlines and dungeons, and things unique to Legacy of the Dragonborn, which is a ton of things. I'm also pretty sure there's compatibility with some major quest mods and stuff, so the artifacts you find in those can be added to your collection in LotD. Wasn't really my thing to collect and display stuff, but I hear the content added is worth it alone.
https://www.nexusmods.com/skyrim/mods/52248/And of course,
there's new lands mods that a lot of people consider must-experiences.Beyond Skyrim: Bruma:
The first release in a series of releases that aims to add the entirety of Tamriel to Skyrim's engine (and they're actually making a ton of progress!). This one is fantastic and has me really excited for the rest, it adds Bruma and the areas around it, including the Wizard's Tower from the Oblivion DLC, and Cloud Ruler Temple which has been apparently transformed into a lovingly made dungeon experience, which I still haven't started but plan to soon. The rest of Cyrodiil, as well as High Rock + Hammerfell, are on their way pretty soon.
https://www.nexusmods.com/skyrim/mods/84946Falskaar:
Falskaar was kinda the original new lands mod. It adds a DLC sized new world to Skyrim with its own stories and stuff. Not something I experienced because I had weird mod order inconsistencies, but something worth experiencing I've heard.
https://www.nexusmods.com/skyrim/mods/37994Shadows of Morrowind:
While not anything super complete at the moment, and not something you should have high hopes for in the future (afaik), this mod adds Morrowind as an explorable area, basically as a way to see the Morrowind world in Skyrim's engine. I'm pretty sure it's not much more than that, but they might have been expanding on it since then. There's other mods out there, in progress, that aim to go above and beyond Shadows of Morrowind, including Beyond Skyrim(: Morrowind), and Skywind. Still, pretty cool landscapes, if you just like to explore, and hey, it might be more than that and I'm just an idiot.
https://www.nexusmods.com/skyrim/mods/35789Beyond Reach:
Totally unrelated to Beyond Skyrim, Beyond Reach is a less than lore-friendly mod that adds some new lands I think west of Markarth. It should still feel like Skyrim, but I think they take some twists and turns that makes it feel different. Can't personally recommend it, and it's not something I was told is a must play thing, but more lands is more lands.
https://www.nexusmods.com/skyrim/mods/48467The Gray Cowl of Nocturnal:
More of a massive quest mod than a new lands mod, GCoN is a renowned mod that centers around searching for the Gray Cowl of Nocturnal from Oblivion, which ended up in Hammerfell after the Oblivion Crisis. Regarded as one of the top mods of Skyrim in terms of content, it's definitely something you should check out if you like a mix between stealth, combat, and puzzles. Emphasis on puzzles--think before you do.
https://www.nexusmods.com/skyrim/mods/64651/Skyblivion and Skywind:
The TESRenewal project is something I've been following since I heard that some dedicated fans were trying to remake the amazing but clunky game of Morrowind in Oblivion. They've come a long way since then, though, and are now slowly but surely making progress on recreating and expanding on the games of Oblivion Morrowind in Skyrim's engine. You can't play it yet, you might not even be able to play it in 2018, but it'll definitely be an experience worth the wait, I'm sure. They're the most professional looking Skyrim mods perhaps ever made. They're remaining loyal to both games, but drastically, and I mean DRASTICALLY overhauling the world space to both make them bigger and make them more interesting and detailed, so traveling all over on foot will actually appeal to more than those who like to stumble across weird quests. Again, no release date in sight for either, but seriously, keep your eyes out for both, they're likely going to be absolutely fantastic experiences even if you're not big fans of Morrowind or Oblivion.
Enderal:
Probably the most ambitious mod behind Skywind, Skyblivion, and Beyond Skyrim, but perhaps for that reason, it's actually out. I think this mod has won awards from places like moddb, and it's so significant it's regarded as its own entire game rather than a mod for Skyrim--it has nothing to do with TES, it's just build in the engine. You unfortunately can't use most mods alongside Enderal, but its installer is pretty awesome, and will put all your Skyrim mods into a backup while Enderal is installed, so that when you uninstall Enderal through the installer, all your mods should be put back where they were. Anyway, this thing is basically a game, which is free, as long as you own Skyrim (and maybe the DLC). It plays like its own game, the world is interesting to explore and just about every NPC you come across is interesting. The dungeons and environments are well paced, so things never feel boring, and the whole experience feels extremely professional. I wouldn't be surprised if the dev team becomes a company in the future, if they haven't already. It isn't TES, but it's one of the best things you could experience when launching up Skyrim. Personally, though,
I recommend playing this before trying to heavily mod Skyrim, because though the installer is great and all, it's not at all perfect, and can make mistakes, especially with SKSE and ENB stuff.https://enderal.com/There's of course also a ton of mods that change the graphics, mechanics, UI, sounds, or add new content like weapons, armors, houses, followers, etc., but those are all up to personal opinions. For instance, after Witcher 3 reawakened my love for the Witcher franchise, I modded Skyrim with some mods that made it play and feel more like Witcher 3. There's also mods that change combat, some of which inspired by Dark Souls, which are things I can personally recommend from my own opinions but they may not be what you'd want. That's the only reason I haven't recommend mods like VIGILANT, which is a pretty fantastic DS-esque quest mod. Check some of the combat mods out, check out some of the graphic overhauls. Skyrim is very easily transformable, and that's its #1 redeeming factor, since it fixes, redoes, and expands everything good or bad about Skyrim.
P.S. I'm a fast typer and I know about these mods, this didn't take that long to compile.