The POD for this Avatar world is somewhere at the end of Legend of Korra.
Kuvira is finally defeated by Korra and the Earth Empire is divided up much akin to the end of WW1 and WW2. 
Omashu; which I'm assuming was never fully incorporated into the EE, but was instead defacto occupied by it. My reasoning is that at one point in S4 of LOK Omashu is still yet to be taken. Not too long after; Kuvira tells us Zaofu is the last EE territory to be taken (besides the UR).
Anyway, Omashu is given a large chunk of the former Empire. Chin and Zaofu are back. The Swamp is given autonomy. The UR's borders expand. And because no one wants the islan