The very simple reason why it wouldn't work is because Warcraft in general and WoW in particular has always put very big emphasis on the heroism. Sure, some friendly characters commit nasty actions (almost all of them faction-based) and sometimes you participate as a player. But in the grand scheme of things, your character has always been the good guy. In raids we are always the force of good fighting against evil, because good must prevail against evil in a hero story.
Herein lies the problem. The Burning Legion consists entirely of omnicidal maniacs, is run by a titan who has been corrupted for eons, and spearheaded by an always chaotic evil species. There is no conceivable way for them to ever be anything but villains of Azeroth, no matter how big the retcon. And as long as any race is nothing but permanent, omnicidal chaotic evil, there's no way they could be the protagonist of a hero story.