I got thinking about this because of
kc_anathema's journal, into which this appeared in modified form as a comment.
The series as shown leaves lots of hanging plot threads. Where is Zuko's mother? (It actually cliffhangers that question, with Zuko asking Ozai but the camera cutting away from Ozai's answer). What will Zuko do to find her?
Are the Air Nomads really extinct? If so, will Aang manage to restore their culture? If not, where have they been hiding? Keep in mind that if airbending vanishes, it may break the reincarnation cycle a few centuries down the road.
None of the three main couples have actually gotten married yet. Will they? There are obvious problems of divergent responsibilities which could divide each of the three. If they do, will they have children? What will these children be like?
How about the South Polar Water Tribe? They were close to extinction (one small village, population way below any recoverable bottleneck) but then a lot of them were captured in the war, and some will be released. What role will Sakka play in this?
The world is clearly on (or a bit over) the brink of a technological revolution, accelerated by the war. What changes will this bring?
Interestingly, it has not yet developed a scientific revolution (many of its technologies are precocious combinations of mechanics with various bending forms). This is important because, if that world develops chemistry and physics, one obvious side effect will be that the bending forms will be able to cross-fertilize.
(both Iroh and Toph have independently made breakthroughs related to this without either grasping the general principle, in Iroh's lightning-redirection by treating electricity as "fluid fire," and Toph's metalbending by treating metal as "altered earth." "Combustion Man" may have achieved a similar breakthrough, but it may have perished with him).
In short, there's a lot of good stuff that could be done with this universe. My hope is that it will be done. My fear is that they'll keep redoing the same things with the same characters, in an element-bending version of the Land Before Time series.