It's not often when people create an 'enhancement' of something decent to begin with and actually manage to improve upon it. This makes it so much better when it happens! This campaign was a glorious experience.
The unit selection is wide and at the same time fairly viable across the board.
- Footmen can fit any role you might want from them - frontline, second line, ranged support, or get tougher for some resources. They do fall on the wayside later on, although such is the fate of a true jack-of-all-trades. The veteran is great at Culling, if you want to protect your home base, but don't want to commit too much wood to it. The mountaneer would probably be fine but it appears too late to have an impact.
- Crossbowman is better for most of the campaign except the very last mission, where a lot of damage tends to bypass the frontline - a perfect balance letting you make an actual informed decision if you want it.
- Cleric doesn't replace Priest as much as he supplements him. The channel heal is okay by itself but it's not good for most of the campaign, since you have access to Arthas who does single-target healing better. Autocasting that makes cleric waste all his mana during repositioning trying to heal people. That said, he brings to the table something unique - Appease Spirit, which goes some way towards denying the corpses to the enemy. On every mission save Culling (where consisntent healing mattered to me more) I tended to use one cleric per squad, along with 2 Priests to do actual healing. Appease Spirit has the unfortunate side effect of denying YOU corpses for resurrect, but in Hard usually if resurrect matters, you did something wrong and are in the process of getting wiped, so it doesn't really matter.
- Bishop is a viable replacement for a Priest as the team's health sustain. You still would want a Priest for his other utility, but pure hp throughput is hard to beat. Problem is, in Frostmourne, where every little bit helps, red locus is pretty mandatory (if you dont use the 'easy mode' tome). And everywhere else Arthas pulls most of the team's healing by himself just fine. Also, having Arthas armed with 'Old Horde memento' staff, Bishop's resurrect thing for 200 mana is extremely situational. His mana is best spent on the living - emergency teamwide heal is gamechanging.
- Cavalier is... I wouldn't say bad. But his time, while clearly defined, never comes. The enemy comes in mixed waves of Abominations, Ghouls, Crypt Fiends, Necromancers and Bone Golems and the heavy armor really only helps vs the former. In practical terms they appear more squishy than Knights, move slower and do less damage. This is clearly a PvP counter for mass Abomination spam but vs AI it doesn't work. Knights of the Silver Hand are somewhat better but they're locked with green locus and an upgrade, and they're not better enough.
- Enchanter is amazing. Being able to remove cripple, sleep or frenzy, or get more mana for that huge value emergency AOE heal really can't be hyped enough. Just need to remember to turn on the autocast

Actually, even Blizzard got the similar idea with the Spellbreaker, but the elite High Elf infantry wouldn't really fit with generic Alliance human-led army.
- Battle mage is fine. He can substitute and be a direct upgrade above a dwarf rifleman, while having some AOE and stun options. It's certainly worth it to take one per squad with you.
- Conjurer is... Interesting. He's the definition of glass cannon, with significant damage output and a mighty elemental summon, but - 300 hp, no armor and no slots to compensate for that. He's also available extremely late, at least his summon is. And by that time, sustain and health pool beats pure damage. You can't really dps race an endless tide of the dead. That said, it's nice to have extra expendable 1500 hp on the field for the final push for Mal'ganis, so there's that. Again, along with the Cavalier, it really feels designed mostly for PvP, not Campaign mode.
- Leutenant is fine. Decently tanky, with two slots (to make him more tanky, usually I picked heavy armor + hp potion), with a nice damage buff and a speed boost active. Available early, they really should be deployed throughout the game, 1 per combat squad. Dwarf Leutenant is geared towards static defense, which is a nice option to have.
- Alliance Commander. Wow. As close to a hero as a mortal can get, with a price tag to match. If you want a frontline that will not break, pick them. When they fall, no one else is left either, so death penalty debuff is irrelevant. And they can fly up to brawl the frost wyrms so you are always prepared. Somewhere an Illidan gets sad every time you deploy one of them. Pure awesome.
- Mortar teams are extremely versatile now, on par with Footmen. Bounce Shot for brawling, Ironfoe Cannon for siege, Ballista for static defense, shrapnel upgrade for general purpose. Anything you need them to do, they are able to.
- Trebuchet. According to description, as impressive as dwarven cannons are, you can't really beat giant rocks slamming from the sky. To me, they seem a perfect tool for Frostmourne, bypassing the need to play attrition with the dead. Find a good spot, get up some ivory towers and a farm, some trebuchets and let the enemy come to you :3 The map's layout also plays in favour of this strategy. They also happen to clear out enemy meat vagons super fast.
- Walls are a nice touch. I do believe locking them behind red locus is somewhat unfair, but since red locus is the default choice for most missions anyway, it matters little. Also they can repell weaker waves by themselves, which is extremely helpful.
- Villagers really should be available with Keep, not Castle. A slightly cheaper repair-only unit would be super helpful in the earlier missions, and not all that much later on. The gold gain from the arsenal isn't good vs AI, you want resources early to power through these first critical minutes, not invest into mid- to late-game income where you are past the shortage phase anyway. Again, this would work much better in PvP.
- Pack horse. The unsung hero of the Alliance, the horse excells at keeping the frontline healthy, charged and protected with turrets for a minor investment of gold. If used right, it can be as vital to the effort as a trebuchet or a commander.
The only issue I had about the units in general is the Siege Engine. The tank didn't have any changes made to it, which means it's now fairly weak in comparison to everything happening around it. Building-type armor used to protect from piercing to compensate for it's relatively low hp, but with it redefined the tank can't really be used as intended - to break through static defenses. If you ever come back to the game, it would be nice of you to show it some love.
The new abilities are great. Anti-siege trait really lets flying machines take their place with the Alliance, and the dream of combined warfare ability comes ever closer. Plated is also amazing, the heavy frontline really feels like they're wearing actual armor... Which only makes the spell damage they recieve feel that much worse :3 The food deployable is great, even midfight the regeneration field can matter. Weapons is more tricky, since it seems to affect both friend and foe, so I tended to avoid it. The spell books are situational, but super useful when the time comes, the traits help shape the fight like you want it to go... Every little bit does something, and that's awesome.
The changes on the maps are great. More neutral camps, more items, more STUFF going on. You can play the map quite a bit more than in the original, personally I enjoy that greatly. The scarab rush on Shores of Northrend was memorable. The Alterac mini-series. The Culling... You took the story of Arthas and improved upon it as well :3 I really accept that as headcanon now. Frostmourne really feels like a struggle, once I had a glitch with both air and land waves arriving at once, nearly wiped two full squads backed by turrets and mortars. I'm not really good at W3 so I had to play that mission at slow.
Overall, really. Flat 10/10. The minor drawbacks just pale before the amazing work you've done here. This is one of those cases where you can't even say 'this should've been from the start', because it really couldn't have :3
I hope you get through with the undead campaign. Human one is a masterpiece, so I wouldn't expect you to surpass it, but even something WORSE than this one would be a major improvement over the original. Archimonde summoning was plenty epic alredy, I wonder what will you do with it :3
UPD: The next day I realised I forgot one unit.
- Royal Guard. Well... The concept is interesting. The thing with it, though, is that they're available at the point where Arthas is by far the most durable asset in your disposal. Not quite one-man army like in the original, but definitely one-man frontline if he could hold aggro away from the squishies. He absolutely needs no help surviving whatsoever, assuming you did at least some of the optional stuff in missions prior to Frostmourne. He can duel Mal'ganis and win, duel the Doomguard and win... What ends up happening as you deploy the guards is, if you're using lore-friendly approach (3-4 well-rounded squads spread around the map for the duration of the game, like I did), the command squad which is supposed to spearhead the asssaults gets too squishy. Light armor really isn't all that good for a melee tanking unit, not at this point. No inventory, so they can't be improved through the gear, they're fairly slim, so it doesn't make them that all that great at holding the chokepoints. Even dismounted Knight makes for a better blocker, not to mention the glorious Alliance Commander sharing most of the same requirements. If you're using gamey approach with endless unit production to overwhelm the enemy, the Knights again come out ahead as more cost-effective fighters. Even in PvP, unlike most units, I don't think the Royal Guard would be all that interesting with how late they are available and how durable Alliance Paladins and Mountain Kings are. The Lichguard with it's active hp/mana restore on a hero and a wider model is much, much better.