Ion just stated that no new Allied Races will become playable at 8.2 Rise of Azshara in yesterday's Q&A.
As major patches lately have a 6 month duration, so i guess only in early 2020 will be the closest expected date for us to get a new Allied Race.
Taking this into consideration, i bet the new Allied Races will be presented at Blizzcon together with the new expansion and the new hero class(es), aka Tinker and / or Dark Ranger (my bets).
When we see the ingame screen selection for new characters and we choose new Allied Races, we see below the current four ones in each faction that it seems to exist space in the blue and red faction stripes where it is possible to fit two more icons / slots of at least TWO more pairs of Allied races.
Is Blizzard going to release on 8.3 (probably fight against N'Zoth) two more pairs of Allied Races?
My bets: Mechagnomes vs. Vulpera will be the most realistic choices.
Unsure about the next pair though.