If you base it off the comics that game out after the first show there is political tension in the aftermath of the Avatar the Last Airbender. There was fire nation colonies in the earth kingdom and both water tribes were crossing the world to get to each other. I think instead of ba-sing-se you do a region of the earth kingdom with the colonies and this means coastal which means water benders could be showing up for whatever reason.
Justification for everyone staying they just got out of the hundred year war and everyone is super on edge the fire nation is going to revert so I was going to visit my granny in the southern/northern water tribe but I see trouble brewing here and I am a 'veteran' of the war so... I'll just hang out a bit.