I think there are only really two ways we could get playable ethereals in Midnight:
- One of the base zones is K'aresh or another location instead of being in Quel'thalas
- All of the work is done to embed them in the story before Midnight's release
I don't see (1) happening because it would split the "continent" in half, and every time an expansion has separated zones with loading screens or other travel between them it's poorly received. I could see it if part of K'aresh or Telogrus Rift were teleported to Quel'thalas, or if they implemented some new technology that worked flawlessly to transition between the areas without it feeling too clunky, like one of the zones having a Void version where the ethereals can live. But I'm not sure how much they want to digress from the elf storyline--(2) would mean we don't need the ethereals to feature heavily in the base story but can still be present in the greater plot without distracting from the elf storyline. We would also know if it's possible by the end of TWW.