Then why yet another major Orc story arc? There are other races in this game.
You're assuming a lot and understandably so. I mean, if you look at the image, it's a bunch of orcs, right? But the story has never been just about the orcs and Warlords also will be highlighting a bit more about the draenei and even shedding a bit more light on blood elf paladins. The thing is, as always, it's a story about Azeroth and the heroes and villains that live (and die even) as a part of its ongoing history.
For the question about the Alliance and "Captain America", keep in mind, the Alliance are going THROUGH the portal to take it to the Iron Horde directly. They're not planning to sit it out and wait until things get too bad first. Since I don't write or develop the story, I can't comment on the difficulty of developing the Alliance story directly. As a writer myself though, I think that writing any story is about really feeling connected to it and taking it where you want it to go. So, good or evil doesn't affect the difficulty in writing it necessarily. It's about making it work in the right context that is consistent to the personalities of those characters. It's putting yourself in the "What would Jaina do if?" etc. mindset.