I think, if Blizzard has brains, they would choose their new races based on popularity.
If we go back to 2006, exactly the same problem has struck the Draenei. Since they were a product of a badly-designed retcon, they had no real roots in the word, and no fans to buy the game just because of them!
I recently did a bit of googling, just to see how many hits I can get with certain keywords (e.g. naga+wow+expansion or worgen+wow+expansion, and many similar). The result was interesting, and did reflect my expectations.
Pandaren were undoubtedly the most popular "hitherto unseen" race. They had at least 5 times more hits (!) than any other race. So if Blizzard has brains, they would surely go after this theme at first place. (of course, the policy at China might always be a problem, such as with WoTLK, but that's nothing new. Chinese authorities complain with every WoW product, no matter what it contains. And the prescribers in US/EU/JAP/KOR pay several times the money a new chinese subscriber brings)
The second most popular race was the Naga. Though they only surpassed the rest with a moderate margin (approx. 1.5x - 2x). They do have a certain "coolness factor" around them, since they are a primarily aquatic race. Problems are the realization of armor on Naga models (already done) and mount riding animations, etc. But there's frankly nothing Blizzard couldn't do about this. Yet it is true, that level designers would have a though job making an underwater city.
The third most popular race was Goblins. Many people dream of trying their skins out. They also have a perfect lore and art background in the game. Potential problems revolve around the fact, that a tiny, tinkering race was already included as playable (Gnomes), so Goblins would provide little new thematics.
The rest of the races were fairly similarly, and lowly popular. Perhaps the Worgen were still best that the other, but that might just be the hype around these masks. Other, sometimes desired races, such as Ogres, Murlocs, Gnolls, Harpies, Draconids, Dryads, Furbolgs, etc. have very-very low popularity. All these monsterous races bring some color to the game, but that's they are: just mobs giving a taste. Most (if not all) of them are neither popular enough, nor thematically suitable to become an entire playable race. Well, UNLESS Blizz wants to repeat the Draenei "hat trick".