I'm not to sure.
Based on rigs left unused:
- We have undead and worgen. In my mind, worgen rig would be dragonman like race lead by Wrathion. Undead would be something related to Calia, Delaryn or, (wildcard), k'thir.
- There's also human rig and kultiran rig, which haven't been repeated since kultirans are a new rig. Rexxar uses the kultiran rig, so ogres/moknathal. And humans, I'm thinking something Calia related as well, the often speculated light undead.
Based on reputations (since they've been used to unlock the races):
- Ankoan/jinyu and Rajani mogu, though I'd honestly prefer hozen or sky hozen over mogu as the mop era race, and they were the original horde allies back in that expac. Tel Abim is an undiscovered island known for its bananas, make it be the hub for the mysterious sky hozen (or so I wish).
So ... one of these pairs is what I'm thinking right now.