So, we know that there's going to be some scuffling over Stromgarde. From Anduin Lothar, to Thoras and Danath Trollbane, many of the Alliance's greatest heroes hail from the Arathi Highlands. The descendants of Arathor are also notable in that they were all depicted as exceptionally large men, even beyond Stormwind's rugged people.
While many of us have argued for playable Vrykul -- and I'm certainly not saying don't still love the idea -- the thought occurs to me that formally reintroducing the Arathi as an Allied Race of humans with a more unique silhouette, might be the perfect "middle-ground", and also create a much more compelling pillar for the Lore.
Think about it; we've had these big, awesome heroes that hail from the Arathi Highlands (or at least the original Arathi bloodline). Warriors even embark to claim Strom'kar, the Sword of the Barbarian King, humanity's first real "king". And yet, there's really only a few we've ever seen ingame, and they're all simply "large Human character-models".
What if you retconned them, so to speak, and gave them a unique character-model?
My thinking; build them somewhat similarly to the upright Orc model, but obviously with Human propotions. They would still be larger than Stormwind's humans, but smaller and shorter than Orcs proper. The idea is that they would be "Arathi Barbarians", less 'civilized' than their brethren in the other kingdoms, but made up by being great warriors.
Of course, for inspiration how they might look, I would look no further than Diablo 3's Barbarians:
https://i.imgur.com/Rn0zo.jpg
https://vignette.wikia.nocookie.net/..._Barbarian.jpg
Because of their relation to the Hinterlands, they might have a more positive connection with the Wildhammer Dwarves, thus explaining why they might have adopted braids and war-paint to fill out some of their Customization options. Due to their barbarian culture, I would imagine they would lack access to any "Holy" or "magic-using" Classes, save for maybe Shaman. I would almost consider Druid, but I guess it would depend more on how their culture was depicted.
Speaking of which, you've got to flesh them out, right?
The Brotherhood of the Horse has been MIA ever since Lothar died. Seeing as the last of the Arathi-descended humans were known as "the Sons of Arathor", that's what I would formally call their "race". I would explain their "absence" by saying that when Lothar fell, and the "Alliance" was formed during Warcraft II, the Brotherhood of the Horse elected to embrace their nomadic lifestyle, and stayed clear of any conflict between factions. Recent events, however, how forced them from their home in the mountains, and now they're fighting to reclaim their ancient birthright, the Kingdom of Arathor.
Or you could make it even simpler, and retcon them completely saying "This is what Arathi people were always meant to look like, we're just making it official now".
Either way, I actually think it could make a pretty strong candidate for an Allied Race. And given Stromgarde is becoming a staging ground for larger conflict, I think it could be the perfect opportunity to bring them into the fold!