With 7.3.5, Blizzard released four allied races if you pre-order the next expansion, Battle For Azeroth. There are two races for the Alliance: Void Elves and Lightforged Draenei; and two races for the Horde: Nightborne, and Highmountain Tauren.
Each of these races played a part in the Legion storyline, and recruiting them is a very organic development in the story. Each race requires a certain amount of reputation and achievements, which you basically have if you've been playing Legion.
I'm really happy that Blizzard chose to make playable scenarios about recruiting each of these races. A long time ago, I complained about Theramore, wishing its destruction had happened in game, rather than in an associated novel. It's heartening to see Blizzard put important story events in game, and give the player a role to play in those events. It feels like the Blizzard of a couple expansions ago would have just made the Allied races available to play, and put the story of how they joined in a novel or story online, or a cutscene.
Perhaps this change was prompted by competition with FFXIV, which is all in on the storyline occurring in game. Regardless, it's a good change. Hopefully the same thing will happen with the scenarios for Battle for Azeroth, especially the triggers for the war. I hope players have their bloody hand prints all over the burning of Teldrassil or the attack on Undercity, or whatever other events happen.
In any case, I created a Void Elf shadow priest, and have been very slowly levelling her up. She's currently level 35. I really like the scaling changes made to levelling, as well as having zone completion tied to the story lines. I've been levelling in Kalimdor, as I have not seen most of the post-Cataclysm stories there. The new levelling is very enjoyable.