Combat and Skills
Overhaul Morrowind's clunky combat with some some welcome changes, like removing the dice roll behind each melee strike and increasing the pace at which you accrue skill points. Now go out there and be a warrior.
Faster Skill Increases
One of the more straightforward balance tweaks available, with Faster Skill Increases all it takes is a single attack or a few seconds of running or jumping to make the relevant skills go up. You'll get sick of the angelic sound that plays with each increase, but you'll also be able to tackle the interesting quests a lot quicker without grinding around in Ratmurder Town forever.
Oblivionized Magicka Regeneration
Even if you're not playing a spellcaster you'll want access to the occasional Levitate to cross a mountain or Mark and Recall to bounce back to a quest-giver. Wizards have to nap a lot in Morrowind though, because magicka regenerates at a glacial pace. If you're not ideologically opposed to the idea of making Morrowind more like Oblivion, this mod borrows its rate of magicka replenishment and honestly it's a godsend.
Whether an attack hits or misses in Morrowind is based on a random roll behind the scenes based on your skill. In a more abstract RPG that's fine, but in a 3D one where you can see that spear hit someone to then be told by the math that it whiffs it can be jarring. With Accurate Attack any blow that looks like it hits actually hits.
The lovely, thunky speed of arrows in Skyrim was inspired by a mod for Oblivion that made them less dodgeable but much more fun to shoot. Projectile Overhaul puts some of the same arrow juice into Morrowind, increasing the velocity of everything you can launch, including throwing knives, shuriken, and spells.
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Install Madd Leveler to take away the worry about effectively leveling and grinding the appropriate skills, as it'll do all that for you. Madd Leveler raises attributes based on which skills you've been using and does it quietly in the background so you don't even notice. There's also Galsiah's Character Development, which is a bit more complicated and can be restrictive if you're trying to play the kind of hybrid character who doesn't fit a single class.
TESIII Sneaking Realism
Each Elder Scrolls game makes sneaking a little less ridiculous, but even in Skyrim we're still crouch-walking invisibly in broad daylight because our skill's high enough. This mod doesn't make stealth perfect, but it does add modifiers based on the time of day, weather, what armor you're wearing, and whether you have a weapon out. It also increases sneak attack damage to x10 so it's worth all those potential penalties.