I don't think the playable Ethereals will be Ethereum/void, if they exist, due to the skins we've seen. Not even Phase-Shifter Azir is voidy, only his minions, and Kyveza.
But maybe that ties into the Midnight class, if it exists, being void related? So if Ethereals are playable they can be a Void ??? and that's how.
I would assume the Protectorate will be the playable Ethereals and Locus-Walker and/or Ameer from TBC will be their leader.