i think the water tribe could had been raiding the other nation before the formation of the modern concept of the 4 nation. Like the vikings raided europe during the dark ages when central goverment was weak.
The earth kingdom i believe was divided into many states and must have fought each other until the earth king unified them. I have a story for this situation…..
the fire nation were once the sun warrior but something happen and they had to rebuilt there new nation under the fire sages, which would later lead to the royal family etc.
now the air nomad are the most Perplexing, why are they nomad, and as we can see airbending can be use in a deadly way. So i belive that the ancestor of the air nomad were also a nation which fought to control the world, or resource, but something happen and the air nomad were formed, and violence was outlaw. It just seem the air nomad culture is kind of unnaturally made, because one of the most powerful form of bending and people are so Pacifist.
And if anyone has notice, airbending doesn’t have any weakness. It doesn’t need a source to use. Water – u need water and you have to pray that the moon spirit isn’t killed. Earth-u need earth duh to bend, and it restricted on land itself, unless you carry rock with you?
fire and air are the only element which don’t need substance to bend but both require Spirituality , like zuko needed to find his momo inner spirit agian after joining team avatar, and air as we see from korra, you need lot of spirit.
But fire has one weakness, solar eclipse, bye bye power for 10 mins. (yeah not that weak cause you do get comet which allow you super avatar power every one hundard year (we think) i hope it isn’t every one hundard.
so from all the facts, air is the most reliable and usable element, but it is shown as a less deadly art. (people who used it havn’t done all the damage that they can with it.)
like how can you really beat an airbender(without technology, and sozin, bloodbending etc) in a fair fight, you can’t hit them and they can just fly around and knock you off a cliff or just use a tornado etc. it the lack of deadly moves that got the air nomad wiped out, but way were any deadly move see…did they do some bad in there past so now they forsake all airbending move that are deadly
If i was the creator i would write a series about how the air nomad were ruler of the world but change and became what they are now. cool right?!
or something close to that.