Love the concept, feels like this is how this campaign should have been
Adds replayability and roleplaying to this RPG-esque campaign.
What it really needs now, would be a balance patch. Many of the abilities are very weak, which means they don't scale well into late game, and are also not balanced well between eachother. This makes some classes way worse than others. For example, Shaman Healer's Q (lvl 4 heals for 800 during 12 sec) is nothing compared to Healing Wave that your troll companion has, and the Spirit Link also isn't that good in 99% of the campaign, I'd wager. Some tooltips need fixing, like Shaman-Elementalist's R (Monsoon) lvl1 and lvl2 both say 50 damage, so it's same as Starfall that the Marksman has, only limited to a smaller AoE.. I didn't test it yet, but I hope it would do more damage on lvl 2, and more damage than the huge AoE that Starfall is.
I think probably all Rogue types are just subpar as well. The Assasin Rogue, the one with poison spells, has 2 spells that are basically the same, and the ultimate is really disappointing (-8hp/sec in an AoE), looks weaker than basic spells.
Another example is Command Aura (40%) vs Trueshot Aura (50%) - the Command Aura here is way better because it'll work on melee units, compared to the 10% more from Trueshot which doesn't works only on ranged resulting it to be an overall weaker buff.
Consider that there are Spell Immunity items, which, of course, are almost a must to collect, but many spells that are cast of self or other heroes, will be useless if you do have those items. So, another thing to think about would be making those buff spells be able to target spell-immune friendly units, same as Healing Wave already does.
What it needs IMO are ability tweaks/revamps that would make the spells viable both early and late game, meaning at least scaling the numbers should be changed, including mana costs.
Unrelated to abilities, I've noticed weirdly balanced enemies in Act2 starting map - some enemies are the same as in Act1 while they should probably have a much higher attack damage, like the rest in Act2 have. I honestly don't know if it's an oversight from the vanilla campaign, but it's something you might want to change purely if you feel like it, as it's nothing really important.
Oh yes, one more thing: Rexxar's stat growth per level - now I'm not sure about this due to lack of testing, but it seems his stat growth might be the same as in Vanilla, which is ok for Strength-based classes, but Agility and Intelligence based ones will not be on par then. As I'm saying, I didn't test all the classes to see if it's tweaked accordingly. For now I can say that my Marksman simply has loads of Str, and very little Agility, despite him being Agility based, and me buying loads of Agility tomes for him
Hopefully you continue to improve on the campaign, I'm looking forward to complete the campaign with the Marksman and then with some other classes as well