In my personal opinion, given how the ethereals look and work, I think we're more likely to see them as one of the most, if not THE most important antagonist race in 8.0, than that we'll see them as a playable race. They still lack any serious customisation option (for example, a face) that would have to be added for them to work as a player race. On top of that, their homeworld has been taken over by a void lord, which would additionally give support to them being a likely antagonist race, given that the next expansion is heavily hinted to be void-themed.