All Allied Races have some sort of connection to one of the current races, for example Tauren -> Highmountain Tauren, so its safe to assume that Blizzard intends to have one allied race for each regular one. With that said, the races currently missing are undead and worgen, and for a long time people thought that Alliance would get the Undead raised by the Light such as Calia as an allied race for the alliance, probably even reclaiming the Kingdom of Lordaeron once more for the Alliance.
While that may still be the case (Calia failing with the Forsaken and going to the alliance with those who prefer her instead of staying on the Horde), there is another possibility: The Scourge joining a faction as an allied race.
With Bolvar the way he is its unclear which faction he would join, but its highly likely that he would join the Alliance. We have received information about the possibility of Death Knights for the current allied races, even the Pandaren, now why would Blizzard go to all the work of creating new models, an entire new starting experience and new lore for the little benefit of adding a class to more races? Because maybe it isn't just that, maybe they are doing it because of so much more. Its highly likely that Bolvar will play a very important role in the new expansion, people are speculating that we will get into the Shadowlands through Icecrown, so it would make sense for Bolvar to prepare a new Generation of Death Knights, this time will all the races currently available. This could also mean that the Scourge could join the fight, or rather we join the scourge on their current fight against Bolvar's enemies.
I know that this is a lot of speculation, but I kinda like this ideas, and I would certainly prefer to get a Scourge allied race than a race raised by the light.