This universe is so ripe for an open-source Elder Scrolls-esque game. Choose your "race", enter the post-Fire Nation war world where the Fire Nation colonists are getting kicked out from their occupied territories and turning into refugees. Xenophobia towards the fire Nation is rampant. Where the Earth kingdom is rebuilding and uncovering more heinous crimes the government committed. There's some subsequent political unrest and civilian movements. The political instability of the formation of the New Republic and founding of the new city. The Water Tribes seek to repopulate the south. Multi-national organizations form. Perhaps a new university that feels the need to locate and preserve precious mystical artefacts lest another war threaten them in the future. Archaeological digs to find the lost library are undertaken, spurred on by the reports of the Gaang. The fighting league starts up.

Meanwhile, you travel the world honing your skills, participating in discoveries, supporting refugees, playing a hand in political stabilization, witness the beginning of the industrial revolution, etc.

It would be a 10/10 open world game.