I think people have the wrong idea about difficulty. In previous games, all the slider did was increase tedium (and it hardly even changed danger, it just made fights last longer). I don't think this is true at all for Skyrim.
Keep a few things in mind:
- Few enemies are armored
- Only the player has improved weapons and armor
- NPCs have limited spell selection
- Perks aren't assigned to NPCs, so they are effectively several levels lower than they're supposed to be
- Bad AI
IMO, raising the difficulty level just reduces the many advantages the player has. Except for a few boss fights, it doesn't even prolong fights much. Yes, it does make them last a little longer, but just long enough to occasionally have fights rather than one-sided slaughter.
In terms of mods that help with difficulty:
ASIS - for smarter NPCs, perks given to NPCs, more spells given to NPCs, and more spawns
A Combat mod of your choice
In terms of combat mods, there are a few good ones to choose from, and all of them make the game more challenging:
Duke Patrick's - the hardest IMO and definitely the most features, focuses on realism
Duel - focuses more on RPG-style play where character level/skill makes a huge difference
Combat Remastered - more action-oriented than DP
All of these make stamina important, and increase the necessity of blocking. All of them making fighting more involved and interesting. But I still usually play on Master difficulty to give NPCs a fighting chance. 