There's one other mod I'm mainly interested in. I'm interested in a mod that makes the player, regardless of level have a chance of encountering a very powerful enemy that they'll have to run away from, unless the player character is very high level, and even then that this character can give even a high level PC a run for their money.

I specifically want a mod where the chance encounter in beginner areas/less lore significant areas is lower, but in more interesting locations it gets higher. I also specifically want the mod to produce more interesting enemies than just bandits. I don't want a bandit or a maurader to be like in Oblivion, I prefer my game scaling where common/average enemies such as bandits eventually become like mudcrabs compared to the player character. I still like the possibility of certain freak encounters though, so that a non-endgame character still has to be constantly cautious, and even an end-game character must keep their guard up at times.

Any suggestions?

Skyrim Immersive Creatures?

http://skyrim.nexusmods.com/mods/24913

It doesn't just add "creatures" - there are plenty of new types of undead (armored skeletons, etc.), higher-leveled draugr and other types of spawns.

Everything is configurable in the menu - there's a purist lore-friendly option if you don't like some of the new enemies, or you can adjust individual spawns on or off according to your own taste.

If you want a challenge, set the difficulty in the mod menu to high - that way the spawns from Immersive Creatures will pose a much greater challenge than the vanilla spawns, leading to greater variety and the need to use tactics to judge the strength of an enemy. Even at default difficulty you can encounter an enemy which can one-shot you if you're not prepared or observant!

Thanks for the suggestion I'll add that to my list of mods to download.

asis optional plugin encounter zones?

http://skyrim.nexusmods.com/mods/18436/?tab=2&navtag=%2Fajax%2Fmodfiles%2F%3Fid%3D18436&pUp=1

or random encounter zones:

http://skyrim.nexusmods.com/mods/28761//?

should work with any high level enemy replacers, since the levels of the enemies, that are spawned depend on the encounter zone level.

so first you want to look for a mod that changes encounter zone levels, next you want a mod that adds tougher enemies (even if done by adding them to the leveled list, this doesn't matter, as long as the encounter zone doesn't level with you.).

There is a mod called daedric warriors that adds random Hostile NPCs wearing full daedric and are tougher than the ebony warrior, they are placed randomly around skyrim and you will encounter around 4-7 of them at the end of the mainquest