It seems as if the Metal clan and what not are trying to make "peace" because the Republic City failed and they're trying to institute world law that covers all nations. I don't think it's about conquering but making all the nations the same.
Hmmm, the trailer does show Kuvira and her army unfolding a banner over the entrance of a village, as if conquering it. I think that, while THEY might now see themselves as conquerors, they aren't exactly giving the rest of the Earth Kingdom much of a choice by the looks of it.
And they also seem pretty earth-centric, or metal-centric.
EDIT: By the way, here's a spoiler somebody found in the trailer. Those new airbender suits?
Is Tenzin going to be wearing them also? Click the link to find out.