First off, unalaq does in a ways play the antagonist but he won't be the main villain. Even though he got his brother
banished he still doesn't want war. (He tried seizing the south without fighting but doesn't want to fight.) I said in a post
before Grey Delisle (Azula's voice actor) will voice a dark spirit that has yet to appear.( I believe it will be soon in republic city) Which I believe is going to be the main villain.
Eska will play an azula role by tracking team avatar (mostly bolin). Asami will help bolin battle eska with
them ending up together. (they've been together a lot and a pic showed them sitting together alone in republic city)Korra
will seek the first avatar's advise for defeating whatever dark spirit she has to face. After defeating the dark spirit I
believe unalaq will go the same way as Admiral Zhao went in TLA book 1. Tonraq will be the one to truly reunite the tribes.