I think you can expect their official answer to be “if the story calls for it, we would certainly introduce a new unique race” … but,
Unofficially, the last unique race introduced were Pandaren, and that was 3 expansions ago. Since then, they’ve added 8 ARs with related rep-grinds. These require more time, increasing the only metric Blizzard actually cares about (MAUs) regardless of what they claim in public (Fun).
I would expect more of the latter before we get more of the former.