Every playthrough I do, I install new lands mods like Bruma, as well as a bunch of follower mods, all of which I never wind up even going to or meeting cuz I tend to stick with some of the simpler followers such as those in 3DNPC or something like that, and can't figure out how to work it into the roleplay that I go to the new lands
I have also never beat the Thieves Guild or Dark Brotherhood questlines even though I have had the game for over 9 years
Collection Recommendation: Constellations - A True RPG
In spirit of getting more active on the site, I wanted to recommend to you guys the collection found on Nexus called "Constellations - A True RPG"
It's a Requiem-based total overhaul of just about everything in the game, with the main goal of increasing role-play opportunity and immersion, while I might add also greatly increasing the difficulty so that the game is quite challenging, and it takes practice to even take on your basic bandits.
I have for reference only explored around the Whiterun area with the mod, and everything feels very different from what I am used to when playing Skyrim with mods. I am about to enter Bleak Falls Barrow in my playthrough and just working through the bandits on the outside during the climb up the mountain proved to be an exciting challenge that I had to retry fighting a few times.
It is massive, with over 1400 plugins, and for me I have found it to be amazingly stable, which is great as the mods totally overhaul everything, and I as a busy person don't need to spend entire days devoted to building and troubleshooting a modlist. All that I have done is add a few extra little mods that looked cool or that I also like to my list on top of the collection, which was of course pretty simple as it was just a handful, and by nature of adding mods to a game already loaded up with other mods that have been meticulously configured, I would recommend that you don't mess around too much with adding anything too complex unless you are a more advanced modder.
The only potential downside is that you need to have the creation club DLC installed in order to play it, so unless you buy it or somehow get the files then you are out of luck on this one. The latest couple of updates to the collection also use the latest version of the game, so there is no need to downpatch, which I personally like, but if you wanted to add personal mods that aren't for this version, then you may need to use an older version or wait for those ones to be updated if it is a big deal to you that they are part of your load order.
There aren't any weird NSFW mods included (ie: no sex mods or anything that would be embarrassing if someone walked in on you playing) and there doesn't seem to be any bloat to it. Furthermore the creator uses some really awesome new mods like Seasons of Skyrim which really changes a lot up during a playthrough as you can see the world really change a lot during a playthrough. Everything feels purposeful, and the creator is still updating it actively to balance and add even more content to the collection. If you are someone who likes a great, challenging and immersive modlist, but don't have the time to build something so massive on your own and want to just plug and play, I feel that there are very few, if any, other collections out there that are at the level of this one. Big ups to the dude who took the time to actually make it.
I think the popular AI overhauls might allow the NPC characters to do that, at least AI Overhaul (https://www.nexusmods.com/skyrimspecialedition/mods/21654) says that it gives AI weather behavior, so maybe that could help you!
In terms of stuff that goes in the backend of the game, increasing the overall functionality I'd say SPID (https://www.nexusmods.com/skyrimspecialedition/mods/36869)
And also Base Object Swapper (https://www.nexusmods.com/skyrimspecialedition/mods/60805)
A lot of the mods that take advantage of these I find to be much better in terms of ease of use and compatibility for distributing new armors, abilities, weapons, etc. And a few older, script heavy mods like Immersive Armours and Wet and Cold have been given SPID versions if you're still interested in using them but wanted less script bloat to your save
Now that I've played with it, I also really like Seasons of Skyrim (https://www.nexusmods.com/skyrimspecialedition/mods/62861) with the Turn of the Seasons or Seasonal Landscape add-on. It's not too hard to install but for me it definitely helped me get better at modding, and in game in is really cool to see Skyrim in a totally new way. I would recommend you get some of those time passer mods like Time Flies (https://www.nexusmods.com/skyrimspecialedition/mods/39426) so that you can see the seasons a little more as this mod will allow time to pass while you do things slike read, smith, or sell stuff.