disclaimer: I do not own Avatar or Korra…. But I want to.
The war with the Equalist's has been over for two months, in that time Korra had restored balance to Republic City, she had stopped Amon, got back her bending and learned air bending as well, and connected to the avatar state, she even restored everyone else's bending, but though the new avatar group thinks their worries and the threats that plagued them are over but… destiny as well as fate are not quite finished.
Though it is true that the City has been saved as well maybe the entire world, but the same is not true for the spirit world, chaos had started at the beginning of the great war and the genocide of the air benders had only added fuel to the raging, hellish, flames of chaos, but it didn't stop when the war had ended it was merely subdued. And then with Amon's rising and his power to take away bending he had made the chaos worse and had awakened as well as angered the spirits... and now some are crying out for blood and dark forces are stirring because Aang went missing ... and Zuko, well he hadn't been heard of for a long time.