The worgen were to be a primal and feral race and we saw how that turned out. The furbolg would basically be furry night elves even moreso than worgen are in questing.
Ethereal's I find unlikely due to the fact that the way the race is does not make for a very customizable race. I guess they could re-do the model or have some non ripped apart Ethereal join a faction but that goes against them wanting to make sure future races are seeded. Sure the worgen got modified to be playable but the 2 models have many similarities.
I do think Blizzard has a harder time thinking of races for the Alliance than the Horde which have always been quite simple with goblins, blood elves and, one day, ogres. I'd almost say no new races would be better so Blizzard gives the races already in the development they need (like worgen) but the reality is that the playerbase likes new stuff and races make for a perfect new thing.