I've got a fairly solid nucleus of an idea for how to tackle adding the two most popular faction-specific races to the game; Namely Ogres for the Horde and High Elves for the Alliance: Dustwallow Marsh.
In a nutshell, post-MoP Varian Wrynn finally acedes to Jaina Proudmoore's wish to relocate Dalaran to the sky above the ruins of Theramore. Having Dalaran as a populated and updated city versus just a floating invisible-walled prop in the sky doesn't matter that much.
Basically the end result of this move is that the Silver Covenant, realising that Quel'Thalas is no longer their homeland, establish a small settlement either in or close to the ruins of Theramore to call their own. These are the Alliance High Elves you play. Conversely the Stonemaul Ogres feel threatened by this encroachment on their territory and request greater aid from the Horde to bolster their defenses and undermine the Silver Covenant's colonization efforts. Obviously in this scenario you're playing a Horde-affliliated Stonemaul Ogre.
Fairly straight forward. Story-wise, the zone is set and phased in a post-MoP setting with a level 01-13(ish) for both races. Each race has their own phase and storyline to play through in "parallel" with some clever inter-connectivity as both sides have their little wins and setbacks.
I haven't bothered working out more specifics than this but I thought it might be a good framework to introduce both races into the picture as playable characters. It might even vaguely fit in with any kind of south-seas-eque expansion as far as having a coastline-facing zone. Bit of a stretch maybe.