I had this idea right? There are a lot of humanoid races in the game, and I guarentee you all of them have been suggested at one point to be playable. Most of the arguments against implementing them is lore related reasons. Well what if the Blizzard store added races, all with the same racials reworded to fit each race, that could be explained as 'Mercenaries that split from their forces to join the Alliance/Horde.' It would allow for pretty much each of the previously added races to be implemented as playable in some way, and allow newly crafted races to be implemented in expansions. It would cost a nominal fee of 20$ or so, and would unlock that race for your account. These store-bought races would each have their own lore-acceptable classes and would not require specific zones, cities, or mounts. You could have something like:
Another problem that may arise is unique models and helmet models, but a lot of them, namely Saurok, High Elves, Frostborn, Jinyu, Yaungol, and Taunka wouldn't even require new models as they share them with playable races. Helmet models would also be tricky, and mounting models with the Naga. Despite the initial issues, I for one would definitely pay for this service should it ever be implemented. What say you?