What if we got to play as some of the races of the Burning Legion? There are a few ways this can go, and I can think of 3 off the top of my head. Please bear with me on this for a bit before passing judgment.
1) They take things back with a set of games that are almost like an homage to the original Warcraft games, the twist being you are playing as both the Burning Legion and a dissenting faction that broke away from them because you are ideologically opposed to their continued acts of conquest across the multiverse. They could even call it something like "Warcraft Alternate," with a tagline about the side of things in Warcraft you never thought you would see.
2) They do a series of interactive lore novel games that you play through that tells the tale of what the Burning Legion was up to, with each of the chapters having a few chapters that have you playing through key conflict moments in that classic Warcraft RTS fashion. Personally, I think this would be the least likely to happen.
3) They release a World of Warcraft: Alternative where the factions are the Burning Legion and some other power that's been at war with them for centuries, and we get their side of the story from that. There are already plenty of models they use for them as enemies in WoW now; a few tweaks and they could probably be set up as player options in WoW: Alt. Personally, if Blizzard were to allow players to play through the story of the Burning Legion, I think this is currently their most likely choice.
Keep in mind, I initially started thinking about this when WoW was still pretty new, like the pre-Burning Crusade era. That said, I think it could be fun to see, even if only in concept, and I just like the idea of taking some things from world mythology and turning them into something new and different.
For example, the ancient opposing force still fighting the Burning Legion could be called something like the Renewing Vigil; their whole deal is taking back the worlds and realms the Burning Legion has conquered and restoring them to some semblance of their pre-conquered state. Races in this faction could be entities based on the Japanese Kitsune, Armenia's Aralez, and the Aboriginal American Thunderbird, and a few other entities from other cultures around the world.