Scenarios:

-OC is an imprisoned Earthbender and helps the Gaang and Haru with the Prison Break (1x6)

-OC is a freedom fighter and meets the Gaang. (1x10)

-OC is living in the Northern Air Temple (1x17)

-OC is one of Paku's waterbending students at the North Pole and starts off sparring with Katara (1x18)

or

-Create your Own! (Season 1)