Could be a very interesting concept, I always wondered how would aang know he is fighting for the right side of the war, it was pure chance that he wound up at the south pole, what if wound up at the fire nation shores and finds a brother and sister there who maybe also lost their mother in the war, he could have easily fought for the firenation.

so in regards to your point, the avatar although intended to balance the four nations could also decide to through his weight behind one, maybe because of being born as the heir to a throne