Unless you're talking about WoW (which I didn't play at all), Warcraft 3 is actually very far from what I'm talking about.
It has basically ZERO "promotion variability", allowing you to promote your Heroes in ONE way ONLY (just in any order of upgrades).
My point is the exact opposite - you only build "basic units", and then "train" them into more "advanced" units via promotions.
So (oversimplified) you can "train" your Warrior (no specifications) to become a Swordsman (Melee) or an Archer (Ranged), and then you can "train" that Swordsman to become a Horseman (Mobile Melee) and that Archer to become a Musketeer (Gunpowder Ranged), VERY OVERSIMPLIFIED.
But in that example a Swordsman CAN'T become a Musketeer, because it's a different Upgrade Lineage, so to speak.
It works the same way in the normal Civ, of course, but my point is that you wouldn't be EVER able to BUILD Swordsmen or Musketeers under ANY conditions - only Warriors.
Whereas unlocking any and all possible upgrades would be done via Technology AND via Resources (AND via Equipment, when finally enabled) - AND via Experience (hence promotions).
So you'd need Sword Fighting AND Iron AND Swords AND Level=5, in order to promote (NOT build) a Warrior into a Swordsman.
And a zillion other such changes, cultural and maybe even religious, but within the same guidelines, lol.
Also, "training" would also be possible via buildings (and "staying in town for X turns", maybe even via missions like what happens to "not really dead" Commanders now), not just via battles.
I'd be so enthused to see this happen, hehehe.