Blue tweet:
How do you feel having Pandaren shared between both factions has worked out? Would you do the same if new races were added?
Not a fan of it overall. We just didn't want to deny anyone Pandaren. (Source)
So there's go my theory that Nagas would be the next neutral race. I mean, the Pandaren Brewmaster was a neutral hero, thus why its race became a neutral race as well in wow. So I assumed that since the Naga Sea Witch was a neutral hero, it made sense that Nagas would be eventually a playable neutral race as well...