Here's my conspiracy theory about the Garleans:

The Emperor is a primal. He doesn't suck up as much aether as other primals, at least not from his surroundings - but he is the reason the Garleans can't wield magic because he absorps all of that energy from his entire empire.

And he wants more. Like all the primals, he wants more and more followers, more and more power. In true Highlander fashion he wants to be the only primal left alive, and the others must be destroyed because they suck up the energy HE wants for himself.

So yeah. 4.0 we are going to throw down with the ultimate primal.