Take a hyur, pull the ears longer, watch them get skinnier, poof, pretty much an Elezen at that point. Even height isn't an issue, as my Elezen female is shorter than most highlander hyurs. Seems to me the it-looks-the-same argument can be applied to most of the races as they are all distinctly humanoid.
So I'll cast my vote for Viera too. If nothing else, it might thin the Miqo herd a little.
I also think it'd be neat if some of the beastmen races were playable, but at the same time I wonder if it'd take away some of the charm if there were dozens of moogles, sylphs and qiqirn running around. Goblins on the other hand... since they're already integrated into the cities, I could see them being an interesting choice. Haha. Especially if the player had to keep to the laws of gobbie names.