Well, I've read many a reviews and theories already so here's mine:
I think this season was leading up to the great reveal of what the "Revolution" is and why every "bady" has made mention of it. Moreover, why once it is started it cannot be stopped (ala Zaheer). There's lots of reasons for this and I'm going to break it down by working backwards. Let's look at Korra's hallucination induced visions.
We see Amon, Unaloq, and Vaatu.
On the surface, this doesn't seem odd. All these folks tried to kill her, and came very close. However, we should know by now that nothing is as it seems, as in the Avatar world as it is in reality.
Vaatu - of c