What Simca said. People think having a creature model in-game is enough to make them candidates to sub-races, when clearly those models aren't even close to players models in terms of customisations. If Nightborne was going to be a subrace, then the model in Legion would have been much more detailed, they wouldn't have juste done a creature module if it was to redo the entire thing for the next expansion pack.
EDIT: Just to clarify, I think old ones like taunkas are eventual candidates because they would have to be remade anyway. Not Nightborne, because they're too recent and were not made "the way the should have if it was intended to make them playable later".