So I'm going all the way.
It's not exactly Dark Souls
OMEGA, YASH, Requiem, SkyRe.
What do these mods have in common? They are massive gameplay overhauls, combining an insane amount of features to turn Skyrim - balancing- and gameplay-wise - into a completely different game. What do they also have in common? They make the game significantly harder.
Now Skyrim is not a 'difficult' game to begin with. Enemies are absolute morons that you can usually pummel into the ground using your preferred killing method without much thought of strategy. That is kinda fun, pretty simple and easy to keep going throughout the entire game by making enemies scale with your level. Though especially late game Skyrim presents with two issues.
Problem #1 - Encounter Zones
The first one has to do with Encounter Zones (EZs) which determine the level of enemies spawning in a certain area (eg a dungeon). By default, all vanilla EZs have some pitiful min level (pretty much always below 10) and their max level is set to 0, meaning they scale with the level indefinitely. Once you've entered an EZ, it locks at the level that it was set to - if you enter it as a level 18 and then return at level 30 once the enemies have respawned, the game will still treat you like you're level 18.
Fortunately this can be easily fixed with Encounter Zones Unlocked.
Of course you will still be able to take on that Vampire lair at level 6 and that will need some more work to fix.
Problem #2 - Enemy Level
Although Skyrim comes with the PC Level Mult feature - which can be used to calculate a level depending on the player character's - none of the enemies actually use it. Going through the list in xEdit reveals that pretty much all enemies have a set level. The highest-level draugr relevant - and those guys carry Ebony equipment by default - are level 40 (EncDraugr05). Now the next variant - EncDraugr04 - is level 21. EncDraugr03 are level 13. See the problem here? Essentially the core enemies of Skyrim become laughably easy once your Dragonborn reaches level 35+ themselves.
How Skyrim handles variable difficulty
The only way to adjust vanilla Skyrim's difficulty is (obviously) by changing the difficulty level. Novice, Adept, Expert, Master and so on simply change the flat damage modifiers, meaning the outgoing damage you deal to NPCs and the incoming damage they inflict on you. Skyrim's version of 'insanely hard', the Legendary difficulty, turns enemies into insane damage sponges because you barely damage them (0.25x damage dealt) while they hit like bricks (3x damage taken).
Of course that does the trick and makes the game harder but it doesn't necessarily make it more interesting.
How the big overhauls seek to address this problem
Changing perks and adding new ones, rebalancing weapon damage and armor rating, giving them new abilities, perks and spells and giving higher min levels to EZs are the standard ways through which many of the large overhaul-style mods seek to change your game. T3nd0 did it with Skyrim Redone and Perkus Maximus, focusing on more diverse and difficult gameplay. Requiem went for improved roleplay, really reinforcing that sense of dread when you realize that your level 5 noob ass is worthless in a sticky situation. Then there are YASH and OMEGA - both released much more recently - that essentially go very similar routes.
As noted before, all these mods make the game hard.
You will find your level 7 self completely outmatched in Bleakfalls Barrow when you encounter a couple level 60 draugr deathlords that can oneshot you with a mild sneeze. You will get your ass kicked by a bandit mage dual-casting fire balls. And if you think you can kill Morvarth, one of Skyrim's most powerful Vampire Lords, at level 20, well. You are wrong.
In all this sounds like a mixed bag to me. Yeah, it makes a lot of sense that draugr are high level enemies, worthy guardians of the treasures that are hidden deep within the Nord's ancient burial grounds. However, gameplay-wise the aforementioned level 60 draugr deathlords in Bleakfalls Barrow make no sense at all. There is an argument to be made that having to leave an area because you realize that you won't be able to beat it, is immersive. This is sold as a feature for many unleveled / static world overhauls out there.
But there has to be a way to make Skyrim more interesting and reasonably challenging without being sneezed to Sovngarde by deathlords at level 5 - right?
A Different Solution
While researching Encounter Zones and enemy levels for the guide, I stumbled upon this comment by u/Sacralletius on the Skyrim modding subreddit. He makes a fantastic point for a hybrid solution where enemies do level with you, but only in a certain range. This makes enemies a challenge for longer than a few levels and we don't need to add tons of different variants with different levels for one enemy type. In my opinion this is a much better solution than flat level increases.
Let's add increased EZ levels to this with these factors in mind:
1. The level must make sense for gameplay, eg Bleakfalls Barrow must be accessible early on.
2. Bandit camps should be easier than Falmer caves. Spriggan retreats should be easier than Vampire lairs.
Then we add more spells and perks to enemies plus (optionally) specific weaknesses and resistances.
Making It Happen
WILDCAT - COMBAT OF SKYRIM
Dramatically improves combat by making it more dynamic and reliant on stamina.
Item distribution redone. No more Glass+ armor as random loot. Many (quests) rewards and artifacts are unleveled. Generally promotes an unleveled world.
ADVANCED ADVERSARY ENCOUNTERS
Gives enemies more perks and spells. Also increases base levels / level range for many enemies and adds higher-level variants. Will be edited quite a bit.
Much needed revamp of creature AI for more natural behaviour.
KNOW YOUR ENEMY
Adds a new system weakness and resistances through perks. Comes with a brilliant zEdit patcher, making the mod entirely optional.
An overhaul of encounter zones and NPC levels I am currently working on, intended to complement the suite of mods listed above.
Additional Balancing Mods
These are all part of the guide so far, only requiring some more tweaks.
With the mods listed above, a bunch of edits on my part and (apparently) a homegrown overhaul to Encounter Zones and enemy levels, I mean to create a modular gameplay overhaul that does not make the game that much harder. Of course, by using the Morrowloot Ultimate EZs or adding Know Your Enemy to the mix, you can make it more difficult if you so desire! But in essence it should be able to play a balanced Skyrim with less boring combat WITHOUT getting your ass handed to you on a regular basis.
That, at least, is the intention.