There's a lot that the creators can do from here on out. I'm hoping that they will get to explore the world outside Republic City and add more of the mysticism A:TLA had. Not only that, but also flesh out the characters too. I felt bad for Bolin and Asami because they got treated like crap. Mako kissed Korra, who Bolin had a crush for, and cheated on Asami. Why is douchebag Mako such a dumbass?

SPOILER!!:
The sub story arc about Tarlokk and Amon (Noatok) being brothers was interesting and intertwined with the main story well. It also ended perfectly the way it did on the finale; both presumably dead while leaving the mystery of them being alive out there.