Interesting campaign. While I do believe that you did really well going off of the base inspiration of the Orc Expansion campaign, I have a couple issues with the campaign.
The first issue is that it is difficult to keep your Main Character alive in the beginning stretch before we are even introduced to Sylvanas. I played through it a few times to be certain of this, but even if you picked a Hunter class, or went Unholy Spec DK it was pretty difficult. My suggestion is to put a couple fewer creeps of a level higher than 1 in that part before before the character meets Sylvanas and slowly begin to add in more powerful creeps as the character grows more powerful.
The second issue that I have is the optional (tirtiary) and secondary main characters. I very much hate to say this, but the reason why the Orc Expansion campaign succeeded was because the campaign's story could be told without Chen accompanying Rexxar and Rokhan. While Rokhan is kind of forced into Rexxar's story by Thrall introducing him, Rokhan is more essential to the story in the sense of why the character is introduced. Rokhan was one of the best Shadow Hunters in Durotar during the city's initial construction under Thrall. From the events of Reign of Chaos, Thrall is already a character that we know well and know that he doesn't exactly trust easily despite him putting blind faith in Medivh's words, Jaina when it came to a demon possessed Grom, and Grom in general. He has had cases that tested his trust harshly and while he was proven right to trust the people that he had trusted, there was a cost to that trust. Rexxar is an unknown variable. While I am sure Thrall believed that Rexxar wasn't likely to do anything bad, he had just met Rexxar and didn't exactly trust him to be able to stay out of trouble, especially the kind that he didn't start. Rokhan, after being introduced, was as much of an intermediary spokesperson for Rexxar on behalf of Thrall as he was someone who was to help steer Rexxar clear from getting into trouble in the beginning to be established city, Durotar. When we get introduced to this campaign's version of Rokhan, it feels more like you are saying 'Here is a second main character to play around with have fun' rather than, 'I thank you for your help with top secret Horde affairs, but I still don't exactly trust you, especially so when it comes to potentially keeping yourself out of trouble although you don't seem like you would be the one to start trouble either'. Ultimately that kind of disillusioned me from actually attaching to that secondary main character. For a potential fix of this, maybe change up the quest that the Master Apothecary gives to the quest of rescuing Fellson and make it to where he offers to join you out of appreciation for the rescue?
Now as for the optional main characters, or the tirtiary main characters, I noticed that you put in at least two characters for us who play it. A complete tribute to the fact that we got to play not only Chen, but Cairne Bloodhoof from RoC once again as well, which technically speaking Cairne isn't even a tirtiary main character either. He is a secondary main character from Act 2 of OrcX because in order to get the aid of him and his Tauren, you have to rescue his son from centaurs, after which he joins you personally out of gratitude for what you did for him and his family. Anyways, where I have issues with them begin here from the following generalized dialogue:
Merphist - "Hello, there. I am a Deathstalker and I need some items in order to enhance my powers. Should you gather these items for me so I can do such, I may be willing to accompany you on your journey.";
and
Grimthos - "Hello, there. I am Grimthos, an Executioner. There are some items that I am trying to get my hands on. Should you get them for me, I would be more than willing to help you along your journey."
Did you notice the similarities? While they may be different "classes" or "Hero-Types", they both needed you to gather items for you to specifically acquire them, thus making them feel like the same character in conversation. Of course, one can argue that Chen had you gather items for him in order for you to potentially acquire him. Well, that is the thing. He is searching for ingredients to put into an alcohol of his own creation. When you gather them as Rexxar and Rokhan, he let's you (Rexxar) try a mug of it and the scene plays out as it does because you (Rexxar) don't actually drink anything other than what nature typically gives you for survival. Alcohol just isn't something that we drink for survival, not to mention it can kill us if we drink nothing but alcohol. To further my case, Merphist asking for items is more understandable in his request for items as he wants to grow more powerful, whereas with Grimthos, we don't really get an understanding of why he needs the items. Thus, it doesn't really make any sense to me that he would actually send you on a hunt for the items. My suggestion for fixing the Grimthos side quests is making their quest match more to his character-type. So, Grimthos is what, an Executioner? Someone that is more Melee and non-magical oriented? So, perhaps have Grimthos send the characters on a hunt for monsters/characters instead? Afterwards, if you decide to keep that part vague for mystery sake, then have him explain that he has been hunting them for some time and they each have managed to survive encounters with him for some time and now that the MC and party have finished them off with trophies in hand, Grimthos feels obligated to aid the MC and party due to honor of word as well as having a yearning to become stronger and more powerful.
Also, another issue that I noticed with the optional character is that for Merphist, I could have the MC and Fellson at a level range of 6 to 8 (8 being the noted level cap of Act 1) by the time I finally acquire him and when he joins he joins at only level 1. This is kind of an issue because it means that as a player, I have to grind his level up and if I acquire him just before going into the very next Act, that puts him at a serious lag. It is the same for Grimthos as well since I noticed that I could have the party of characters at a level range of 10 to 12 (12 being the noticed level cap of Act 2). This is some serious grind power lag. In OrcX when you acquire Chen, it didn't exactly matter if you did practically everything else, Chen came into the party at the average/lowest level that either Rexxar or Rokhan holds. I don't know if this is a bug, or an unintended oversight, but my suggestion to fixing this is to bring the new characters in at the party's average level or in the very least for a more challenging campaign, the lowest level in the party range. So if MC is level 8 and Fellson is level 7, Merphist is going to be Level 7 (Eight plus seven is fifteen, which divided by two, because there were only the MC and Fellson before the Merphist joined the group, would be seven and a half. However, we only do whole numbers and we shouldn't make things too easy for the players so we round down to seven only.). Whereas for Grimthos on top of presuming that you have Merphist in your party, you could have a party whose levels are 12, 11, and 10, and so Grimthos' level would be either 11 or 10 (11 is presuming you are going on the average as the sum of 12, 11, and 10 is 33, which gives the average of 11 because of the 3 numbers in the party's level range. 10 is presuming that you want to continue challenging your players with increasingly difficult odds.).