We should ally with the Garleans

[Daralii]

That's a possibility too.
But assassinating the emperor only creates a vacuum of power for the evil ones to take advantage of.
We need to ally with the rebels first before doing so.

Assuming there are rebels. There are soldiers that disagree with their COs or Varis, but there's no guarantee they'd be willing to openly rebel, especially to the benefit of theistic Eorzean savages.