Thinking from a lore perspective, adding in any new races makes no sense. The non-tempered beastmen and the human races are on shaky terms at best (with only a small handful allying themselves with the human races), and there are many more that have been tempered that are always hostile. Introducing any new races would require heavy explanation of where they came from, why Eorzea is somehow allowing this new race of people to march through its lands completely unchecked, and why members of this new race would be willing to fight under the Eorzean banner against Garlemald.