Part 5 - Content Mods

Gameplay & Levelling

Wakim's Game Improvements - Modular by Wakim

Drastically changes many elements of the game.
Includes flee AI, a magic system overhaul, dialogue and faction-relation enhancements, and rebalancing of numerous items / characters / game settings.

These changes are divided across 8 plugins, to be enabled as desired:
- Balance - Character: Balances experience gain, races and birthsigns.
- Balance - Game Settings: Adds flee AI and tweaks a number of settings, e.g. giving NPCs more magicka, making diseases more likely, and increasing the difficulty of lockpicking.
- Balance - Items: Balances item costs and stats, improves consistency between items, and adds enchantments to several overlooked artifacts.
- Balance - Magic Effects: Changes the costs of most spell effects to make them more balanced.
- Balance - NPC Spellcasting: Adds new spells that will be picked up by most NPCs, designed to make them less stupid spellcasters.
- Balance - Spells: Tweaks most of the game's spells to make them much more useful, and updates the casting costs based on Balance - Magic Effects (required).
- Gameplay - Dialogue: Fixes a few dialogue bugs, makes some generic dialogue less common, and makes NPCs' behaviour more consistent with their dialogue, e.g. they might run away from you if you are a criminal or diseased.
- Gameplay - Faction: Changes guilds to be more stringent in their requirements, and makes most guild members refuse to serve non-members.
An all-in-one version is available here, although this apparently requires cleaning.

A noteable alternative is BTB's Game Improvements, which expands on Wakim's Game Improvements and also takes inspiration from HotFusion's Economy Adjuster. I have not used this personally, but I understand that some of its changes are very drastic.
Better Governing Attributes by Androl

Changes the governing attributes of several skills to ones that make more sense.
Fair Magicka Regen by Smoke & GlassBoy

Makes Magicka regenerate over time based on willpower.

For alternatives, see Gluby's excellent Comprehensive Guide to Magicka and Spell Effect Mods.

Part 5 - Content Mods

Gameplay & Levelling

Wakim's Game Improvements - Modular by Wakim

Better Governing Attributes by Androl

Fair Magicka Regen by Smoke & GlassBoy