Based upon observations from watching avatar that i started with Air Avatar Yangchen, which is the earliest i could use for a pattern. Yangchen was an Air nomad and a female, Kuruk was male water tribe, Kyoshi was Earth kingdom female, roku was fire nation male, Aang was air nomad male, and korra was water tribe female. Now. My predictions are possibly wrong. that left a future earth kingdom avatar and a future Fire nation Avatar. I used my best answer for balance and prediction. But as said. i could be wrong.