The Sylphs make it sound like the true danger are the primals, and that the Empire is a lesser evil compared to them, which is why they're unwilling to counter-attack by summoning their own primal when the Garleans attack the Grove.
If I were to guess, it's either going to be everyone vs. everyone or beast tribes against our city states while the Empire patiently waits to deal the finishing blow on whichever side comes out victorious (and weakened). The Empire has been trying pretty hard to get the city-states to hate the tribes, which makes me think they're trying to use us as the cannon fodder in this war. Also, as Celeras mentioned, in one of the missions, one of the eikons makes it pretty clear that they aren't friendly towards our kind, and our cities will be closest thing to them when they are fully summoned... so it doesn't seem like our cities will ally with the beastmen.