At least for me, it seems more logical that these are new races that sub races for a few reasons:
It's easier to add a new race to the game in the already existing system while for a new way of customizing, they would need to change the character menus.
I believe that if blizzard would add the concept of sub races, they would add it for every race, and not for a few ones.
<Personal question> What would/should be the difference of a race and a sub race past the character creation?