So since Blizzard said they have more planned, new allied races might potentially be introduced throughout BfA's patch cycle. So which potential races could possible fit the narrative? Also, it seems that allied races must share a skeleton and be some sort of variant of an existing race. This means things like Saberon, Ogres, or the new Fox People will probably not become new allied races, but then again Blizzard can be unpredictable.
That being said, these are the races that I believe have a strong chance of making it into the game.
Horde
Nathanos Blightcaller Undead, lets just call them Blights for short. With Nathanos having new model yet again, it's not crazy to assume we might get a new forsaken group based on his new transformation. Perhaps these will be an elite group of forsaken which undergo the same process Nathanos did to become less decayed. This way the "human" model is introduced to Horde side.
Draenic Orcs is another possibility. Despite WoD's shortcomings, it is still a part of the lore and when the Alliance and Horde are scrambling for allys it would be silly to overlook an entire Planet of them. Brown orcs have always been heavily requested so this would be the perfect way to bring them. Black skinned orcs could also be incorporated as a skin option since all Draenor Tribes would be a part of "Draenic Orcs".
Alliance
If the horde would get another alliance model then it'd be only fair vice versa. Now the only other Horde racial variant that I could imagine would join the Alliance would be Gilgoblins. When we fight Azshara in 8.2, the Alliance could possibly save some Gilgoblins that are enslaved by Azshara's forces and then have them pledge loyalty to them.
As for the 2nd Option I'm torn. I believe Mechagnomes would be an awesome addition, having a mecha race similar to Wildstar would be pretty neat but I'm not sure how Blizzard would make it work with armor. Another Option would be Leper Gnomes.