So we can basically infer the next batch of allied races to be announced will be...
1. Draenor Orcs
2. Vulpera
3. Kul Tiran (Bulky) Human
4. ???
Sethrak have been brought up as a candidate, but the Horde will apparently work with a rebel group to bring down the sethrak Emperor. Tortollans have also been proposed as a candidate but they seem to also live on Zandalar and work with the Horde to commune with Torga, a turtle loa. Thin humans are also a proposed candidate, but that would be extreme human overkill.
So based on what we know, my prediction is the Alliance will either get broken or quilboar.