[Tiva]

Well you also have to clean up the Equalists, and Lieutenant may try to take over for all we know. She has to restore EVERYONES bending that was taken away, and maybe the Earth Kingdom will need some help or something. If Amon isn't dead and he washes up in the earth kingdom trying to become someone else.... Possibilities are there.
As for character development, it was rushed point blank, but the time between the scenes makes the season about 6 months and we barely saw the tip of the iceberg in them. The love triangle was interesting, as was making the Avatar older, but it gave them less time to grow as people.