The thing I always wondered when looking at the world map they show is if it is supposed to be a distorted hemisphere or a really distorted globe.
If it's the whole globe, then is the west coast of the Fire Nation really a short sail from the east coast of the Earth Kingdom? Wouldn't that kind of put the Eastern and Western Air Temples... sorta close?
If it's just one hemisphere, then there's another entire hemisphere for some Air Nomads (on sky bison) to have escaped to at the beginning of the war.
And who knows what else is over there? Could've made a whole Book of Air season out of that, whether you find Air Nomad refugees or just search for them in vain and find wisdom instead. I guess it's too late for that, though.