If you guys would allow me further speculation on the less likely end of the spectrum, I've been thinking of another alternative.
Do you guys remember these?
They are K'thir, evil void cultists from stormsong. Probably just villain fodder.
https://wow.gamepedia.com/K%27thir
But I've been thinking...
For one, they use the undead rig and even model wise it's built from the undead model. Which means, from a technical standpoint, they could be playable.
But most importantly, I've been thinking about Sylvanas, Xalatath (Kaifu) and Ashvane.
We know that Ashvane was at one point the final boss of Siege of Boralus. And with the heavy hints that she was working for Azshara, many assumed she would appear as an Ursula inspired octopus lady at the end. Though that was later scrapped and an actual octopus became the final boss.
We know Sylvanas rescues Ashvane and she intends to use her for something. We also know that Xalatath (an old god blade) will fall into Sylvanas' hands, who tells us she will use it to her advantage in the war. Now this could amount to many different things of course.
But one of the ideas that popped into my head was that we could see an octopus/void Ashvane lead an allied race of K'thir on the horde side, maybe swayed over to the horde by Sylvanas with her alliance with Ashvane and the power of Xalatath.
It would sort of fit in terms of parity, if Horde get an allied race from Zandalar (Vulpera) and an allied race from Kultiras(K'thir), and the Alliance gets an allied race from Kultiras (Junker Gnomes - I have a suspicion Mechagon was meant to either be a part of the continent, or underground) and Sethrak from Zandalar. Shrine of the Storm and Temple of Sethraliss sort of match and could become race hubs for K'thir and Sethrak.
I mean, I know it's very, VERY, unlikely. In an expansion where we'll fight old god servants and probably N'zoth, it would be very awkward to be playing a K'thir. But I had to share. It's definitely not a hill I'd die on.
With the Alliance having Void elves and Lightforged draenei, there's sort of a theme that void or light don't necessarily make antagonists/protagonists, and don't invalidate playable races. So with a lack of a distinctly void race in the horde, why not K'thir? :P
Again, it's a crackpot theory, don't be mad at me. I just enjoy speculating