My picks would probably be...
Arathi Barbarians - Built using the Orc skeleton, they would be noticeably taller and more muscular than Humans, although still clearly "human". Explain that some of the Arathi tribes hid in the mountains of Arathi Highlands, and are now inspired to fight for honor and glory to reclaim their once homeland, Stromgarde. Kind of a middle ground between "playable Vrykul" and recognizing the current Vrykul models are super old now.
However, since Kul Tirans have basically made this impossible (there's no way Blizzard would add TWO human allied races), that leads me to...
The Mogu - However, instead of being based on the Draenei skeleton (as they were in MoP), I would instead have them based on the upright Orc skeleton, but with some notable alterations. A bit more "lean" than Orcs, I would give them a much more upright demeanor, sort of like their original concept art seemed to showcase:
https://d1u5p3l4wpay3k.cloudfront.ne...200px-Mogu.jpg
Plus, it would be great to have a "serious" race that also fits perfectly as a Monk.
Those Cthulu-looking dudes from Kul Tiras - Using the Undead models, but taller and more upright. Sort of "noble and aristocratic" if you will, a race of cthulu-like guys that feed off of others' madness and paranoia. To make them more interesting, I would limit them PURELY to magic-using classes, save for maybe Rogue. I've always been fond of the Mindflayers seen on this cover-art of an old book...
https://vignette.wikia.nocookie.net/.../58/Exile2.jpg