It isn't possible for Sozoun and Roku to be the same person. For one thing, Roku is dead and Aang is alive when Sozoun was still plotting the war. How Roku came to know everything is another matter. Maybe since both Roku and Aang are the same person, Roku automatically knew everything Aang had seen but had a better idea of what was going on than Aang did? I dunno...
However, a proper motive for why the war started in the first place is definitely something I'd like the series to touch on eventually. The war seems to be allegorical to the war between Japan and China in the early 20th Century, with the Fire Nation (aggressive naval power) as Japan and the Earth Kingdom (huge continental country fighting defensively and ineffectually for long periods of time) as China. Japan's motivation was its lack of natural resources, so maybe the cause was something the Earth Kingdom did, considering that the Fire Nation doesn't seem too capable of growing its own food. There might have been some kind of trade dispute where the Earth Kingdom was chafing under its obligation to supply resources to the Fire Nation when the Fire Nation had precious little to give in return, a dispute which Avatar Roku defused (presumably with charges of "favouritism!" coming from the Earth Kingdom crowd).
When the Avatar disappeared, the Earth Kingdom could have used the opportunity to cut off its exports to the Fire Nation, and so the Fire Nation, with its boundless superiority complex, decided that it wasn't going to take this kind of abuse and plotted a terrible revenge, using the once-in-a-century passing of the comet to its advantage, killing off the airbenders so that the Avatar couldn't return and make them give back the territory they conquered. The short-termism of the leadership, however, led to a long and protracted conflict where the goal had morphed from simply securing natural resources to genocidal world conquest. Wars are like that, y'see.
And the question of Aang gathering the spirits before him within himself is a bit of a moot point since by definition he IS the spirits before him gathered in himself.
Although the story is supposed to end after mastering fire, I kind of hope they make the ending a more multi-faceted affair. For example: there could very well be some 'final test' that Aang needs to face before he can be said to have mastered each element. During the course of the series, he masters each and every element BUT...how about, as soon as he masters fire, with a showdown against the Fire Lord (or in all probability a suped-up Zhao) approaching, they are all dismayed to find that Aang never entirely mastered air!? That the final, vital, test never took place, and in fact he's been gradually losing his skill since he's been concentrating on all the other elements? That would be a definite "against all odds" battle if the Avatar has to go in without being a full Avatar. But the important matter would be the aftermath...
Final book: air. Taking place in the autumn. Iroh has installed Zuko as the new Fire Lord and peace has been declared. Is that the end of it all? Heck no...during the War the Fire Nation sent millions of settlers to the conqured Earth Kingdom lands to set up colonies. Over the course of 100 years these colonies have taken on a culture and identity of their own, whole families have grown up in these cities, and as far as they're concerned, these are their homes. But to the displaced Earth Kingdom refugees, these people are trespassers and thieves, and they're going to be out for revenge. Despite the protestations of Aang and co., sectarian violence ensues, and there's little Aang can do about it...because he's not a full Avatar yet.
So the last book (season) could be of Aang trying to find out all he can about his ancestors and their secrets, to get in touch with his heritage and find out what he must do. But now, there's no one left to help him.
I dunno...I just think it'd be rather fascinating to have something start off as the Battle of Middle-Earth and have it end up like Sarajevo.

A well-placed injection of harsh reality into a fantasy sphere. Things don't have clear-cut endings...only compromises.
Bookmarks