This is not a "next expansion debate" thread. This thread is assuming burning legion is the focus of the next expansion.
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It's tough to say. You could say that races are added in part to an expansion's base content (i.e BC and Pandaria), but Cataclysm was a little strange. Worgen definitely came out of no where. So did Goblins, they were practically written in already to a new story.
If I had to guess, I'd say..
Ethereals either neutral or a horde race. They are similar to goblins imo.
Alliance is tough. Worgen came out of no where and was a randomly added race. Could be anything.