First, Zarah'Shar. If your intention is to make her an unlikeable character - congratulations, that certainly did it for me. Although I'd say she's not just unlikeable, but also somewhat bad in terms of general character design.
I don't like Zarah'Shar, she's just too bland and feels like some kind of Mary Sue. Failed at her magic studies, and then suddenly got demonic powers served for her on a silver platter? And not even a usual kind of shady deal with demons, where there's some catch and you end up being completely fucked by that kind of deal. Nope, she just got it for free, no strings attached at all. Also no lore for how exactly it happened either, and dreadlords don't even care about that as if humans assuming a demon form and assimilating demon powers is a common thing.
And what's with her motivation? Parents considered her a disappointment and she just went "fuck the whole world" mode? Really? That's some
Hatred level of edgyness.
And that chapter 7 ending cutscene... Baroness encouraged Zarah'Shar throughout the whole campaign and that was enough of an evidence to conclude that Baroness is her sister? Really? Well, with that level of deductive thinking she might as well conclude that Mike is actually her long lost and mutated brother.
I mean, prior to being recruted by the Cult, Zarah'Shar was virtually nobody, considered a disappointment by all important figures of Kirin Tor. Then suddenly some cultists approch that failure of a mage and soon she even Kel'Thuzad in person, who offers her to meet with Baroness, the very leader of that cult. And all that sudden attention to her persona didn't seem suspicious, didn't set her deductive genius on a path of discovering Baroness' identity. But a bit of "kindness" and encouragement from Baroness was enough to deduce her identity.
Antonidas may be a grumpy old asshole, but he is 100% right about calling Zarah a whiny brat. So in the prologue she mentions how she's disappointed by the lack of actions from nobility and how they mourn Arthas' death for 2 months already, while common people suffer from hunger and other hardships. And then, when she's about to kill Antonidas, she whines about her sister being the only person acting kind towards her. And 0 fucks given about all the lives she ruined. So basicaly the same attitude as that of Lordaeron's nobility, which she hates and blames so much. A hypocritical whiny brat.
Now, the "what if..." part. The campaign description mentions only Arthas' death as major difference over original W3 story, so what I expect from this alternative story is that it would develop from that exact difference, and everything else before that point would be the same. If your story actually includes some other changes of original story prior to Arthas' death, then I suppose my negative reaction in that regard is mostly unjustified, but these other potential changes aren't really explained or even mentioned, so I consider them plotholes.
1) Plague, hunger and other illhappenings in Lordaeron. It's mentioned in Prologue, mentioned in dialogue with Clay, with Antonidas, and a couple more times in some other dialogues. So... What plague and hunger? It's mentioned multiple times, but never anything concrete. Like, how severe is it, how did happen, what caused it? Lordaeron wasn't exactly a Heaven on Earth, but it was far from having some major problems like that either. I'd assume that plague and hunger were the work of Cult of the Damned, but the story here implies that Arthas' death ruined all original plans of Lich King and so he decided he'd have better chances in Kalimdor. So it wasn't the Cult's doing. And since nothing else mentioned in the original story or this alternative story might be the cause of such kingdom-wide problems, verdict is - plothole.
2) Orcs. Grom's dialogue mentions that he is not a warchief and that the warchief wouldn't like that he's about to kill Arthas. So I assume, as no other changes of original story were mentioned, that Thrall is warchief. Naturally, Thrall wouldn't permit to take a small army of orcs to raid and pillage nearby human settlements. Yet Grom brought enough orcs to even overcome Garithos' forces.
3) Why were Garithos' forces so few? In the original story, the pitiful state of Lordaeron's army was the result of opposing the Scourge. But here the High Death just went for Kalimdor instead, leaving Lordaeron virtually untoched.
Also, about that cutscene of Grom's orcs vs Garithos' forces. Garithos is incompetent xenophobic idiot, but I believe he's seen his fair share of action, enough to know at least that a proper army should fight in specific unit formations. Well, at least as much "ranged units at the back, melee units at the front". And they knew that orcs were coming, so surely defending forces had enough time to form proper positioning. But the cutscene shows just a handful of units standing on their positions when the attack begins, and lots more running to frontline from the back of the camp, in complete disarray, as if they were taking a nap and suddenly woke up like "oh shit, we're under attack". 0 discipline and strategic knowledge from alliance forces. Surely they're not that incompetent. That just looks silly.
4) What the fuck is even Lich King's motivation to invade Kalimdor over Lordaeron? Kalimdor, half of continent is deserts, sands and rocks, a territory that is hard to conquer even in terms of logistics, and the other half is forests inhabited by night elves. Elves who lived thounds of years, are extremely expirienced figthers, with huge advantage of fighting on the territory they have complete control over and know every square meter of land there. Even defeated Legion once. And had a literal half-god on their side. And meanwhile Lordaeron is dealing with orcs, hunger and plague, the whole kingdom is collapsing even without undeads' help. I imagine the Lich King's thought process was like: "Lordaeron? That's too easy, I didn't spend all those years mastering my craft to pick an easy difficulty. Now Kalimdor is a worthy challenge".
5) Okay, let's say the power of Well of Eternity is what lured the Lich King. But dreadlords are still a thing, how did they miss everything that led to that point? I mean, invading Kalimdor isn't something you do during your lunch time, so how did they only spot it when High Death was halfway done? But okay, better later than never, right? So how did they handle a threat of sabotaging the whole plan of Legion's invasion, a threat of someone else taking the Well of Eternity, the very reason for the whole invasion? They sent one (1) dreadlord with a bunch of orcs. They just said that their (whole) forces in Lordaeron are not enough to oppose the Scourge and they sent only 1 fucking dreadlord with a bunch of orcs to do deal with a problem, that couldn't be dealt with by their entire forces. Bear in mind, dreadlords are supposed to be highly intelligent, masters of plans and strategies. Not even a single puny demon came through the portal along with the orcs.
Oh, and Malganis died like a complete chump, not even some cutscene or dialogue. Just got defeated and forgotten, that's it. Shame.
6) Same issue as was in BS and FK - how did the High Death accumulate enough troops to deal with the elves? So High Death consists of 3 "factions".
Harpies were acquired at Kalimdor, and they're not really that strong or numerous.
Undeads simply can't be that numerous - in the original campaign Scourge's countless troops were the result of ravaging the Lordaeron + troops from Northrend. Neither is the case here, since all original plans concerning the Lordaeron were ditched in favor of Kalimdor.
Skavens are the only "faction" whose potential numbers and might are justified by the story.
Now, having a huge army is one thing, even if it wasn't so plothole-y, there'd still be 1 question. How did that hypothetical army sail to Kalimdor? They did they get enough ships? Another important detail left in a state of plothole.
And even if I omit these questions, I still find it rather unbelieveable that High Death managed to stop NE like that. It took a full-scale invasion of Legion, deception of Grom's orcs and Scourge forces to lay waste to NE. And even then NE managed to win. And yet in this campaign all that and even more was achieved with by a bunch of rats, flying hags and 1 whiny brat with a bit of undeads? I don't even...
7) Speaking of overcoming elves. Cenarius' death was... Actually, I'd say it's one the least questionable moments. The idea with corrupting the earth to weaken him is nice. Gameplay-wise, it felt off that he was just standing there passively, and NE overall played strictly defensively here. That is, this mission being non-build, leaning to RPG playstyle instead of RTS like in the original campaign, is not something I'd prefer, but it's more of a personal preference, so whatever.
However, the way this plague was introduced is plothole-y. Like, what is this plague even? The plague in the original campaign is the plague of undeath, it has nothing to do with corruption the earth. And there was no mention of any other plague being developed specifically as a tool for invading Kalimdor. Especially so potent that it took so little time to weaken Cenarius so much.
8) So before High Death acquired harpies and brought in all its forces, Cenarius and Tyrande were willing to bring down the very stars upon any intruder. They were not just ready but fully capable of destroying (or at least heavily damaging) a small army with their starfalls and all. But when the High Death commenced the full scale attack, both Tyrande and Cenarius apparently forgot what they're capable of.
9) Cenarius sending Tyrande off to defend Hyjal is some B-tier horror movie characters' logic. You know, when in such movies characters decide to split up for no sensible reason and then they all end up being killed.
10) You know, story being bad on its own is one thing, but story being disjuncted with gameplay is probably even worse. So Zarah absorbed Well of Eternity's energies, the most potent source of power on Azeroth, a source so potent that it became Legion's main objective to obtain it, so desirable that Archimonde, basically #2 in terms of power in the entire Legion (#1 being Sargeras), wanted it for himself. And Zarah, a mere human that couldn't even handle her magical studies at Kirin Tor, got it all for herself. I don't even question how a puny being like her was able to absorb it.
The main issue here is that such a major story point has virtually no impact on the story. So she got all that immense power, and then all she used it for is... Nothing. She just bragged it being most powerful mage on Azeroth and that's really it. All other feats of her are no better than what she was capable before. She still needs her army, she still has to resort to some dirty tricks instead of effortlessly overcoming everyone and everything, like would be expected from someone so powerful, and somehow she's even intimidated by Khadgar? I mean, she just destroyed night elves, virtually the most powerful faction of Azeroth at that moment, and she did it BEFORE absorbing Well's energies. And now, after becoming immesurably stronger, she can't even purge Lordaeron without Mike's help? That very Lordaeron that can't even handle orcs, and is doomed to fall on its own due to plague and hunger (especially hunger after both orcs and High Death took care of grain reserves)? The only logical explanation here is that she is so incompetent that she doesn't even realise the real scale of power she now possesses and doesn't know how to use it. And considering she failed even at her studies in Kirin Tor, that explanation seems fitting.
And remember how in the original campaign after Antonidas' and Medivh's dialogue, Jaina tells Antonidas that she sensed immense power in Medivh? So that means that mages can roughly sense the power level of other mages. And here Zarah absorbed Well's power and not a single mage in Kirin Tor sensed anything. Guess her theatrical skills are just
that good. Should've just had a career of an actress, surely it would be leagues more successful than her career of a mage.
Now, the actual gameplay. She feels weaker in part 2 than she was in part 1. Seduce and Summon Shivarra are the same. Invisibility, despite its great tactical potential, feels weaker than anti-magic shell. Raise Dead is the same spell that ordinary necromancers have. She raises tha same ordinary skeleton that any other necromancer can. While Implosion is a unique skill and, unlike Raise Dead, actually gives the impression that it could be something that Zarah got after absorbing Well's enegrgies. Burn Life is cool and strong, but it's nowhere near Blizzard, which is capable of leveling down the whole armies and bases almost entirely on its own. And also the lack of powerful artefacts, that she had in part 1, also makes it feel like became weaker.
11) Oh, and what the fuck was Maiev's problem? She does know Zarah absorbed immense power from the Well, she does know that Sentinels are no more and NEs are fleeing south, she does relise that there's no way she can win it. Yet she does the stupid thing instead of retreating and regrouping in the south along with other NEs, to have at least some chances. And after being crushed, instead of fleeing (you know, Warden with her famous blink that enables her insane mobility and elusiveness) she simply stands there and accepts to get horribly tortured to death?