I don't understand the reasoning behind faction specific allied races from a lore POV for most of the cases. I guess we need to wait for the story.
Both the Horde as well as the Alliance helped Mayla in Highmountain and the Nightborne in Suramar, the Lightforged Draenei and Void Elves in Argus. So make these 4 faction independent like the Pandaren and have them choose side on an individual basis.
I can understand the Zandarli Trolls and the Dark iron dwarves being faction specific. So keep these 2 as is.
This way horde gets some Draenei and Alliance gets some Taurens. Also effectively ends up with 5 allied races per faction with cross overs. More options for everyone.