The finale ended with everything mostly tied up, but I am going to guess what issues the second season might deal with.
After reading posts about people explaining that Amon may have been the leader, but with Tarrolock's actions during the crisis and still feeling oppressed there may be a few bender racists who want to keep the fight going like Mr. Sato, except without Amon's bending locking he could go genocidal. Korra could have to deal with making peace between the bender and non-benders.
Industrial age that the Avatar is going through could be disrupting the Spirit World or displeasing the spirits. In the original season the Fire Nation's advancements lead to polluting waters andde-forestizationand in some case angered spirits. Korra herself is a template for how the current era has lost its connection to the spiritual world. Bending is becoming less spiritual in general and more physical sense. In a word it is becoming out of balance.
Unfortunately it will probably deal with Korra balancing out her relationship with Mako and her duties as Avatar and a strained freindship with Asami.
Asami dealing with the legacy of her father.
Bolin living in the shadow of his brother.