This isn't about the movie. This is about the sequel.
The sequel that takes place about 70 years after the ending of the first series, with Aang dead. There's a new Avatar, a girl from the Southern Water Tribe. She has mastered three of the four elements already (Water, Earth and Fire), and is now seeking an Airbending master
That someone has revealed to be Aang's son, Tenzin (
a meaningful name, eh?).
Other interesting things that have already been mentioned include "metropolis powered by steampunk-type technology", the fact that the city is "inhabited by people from all nations", and that some of the problems the new Avatar faces will be "rampant crime and an anti-bender revolt."
Steampunk, people (and possibly benders) from all nations working together, bending-powered supercriminals, more badass non-benders... That sounds quite promising, doesn't it?
Also, since it's just 70 years to the future, we have to consider few things. Think of Bumi, Iroh, the Fortuneteller, Fire Lord Sozin who started the war at an old age, Master Pakku and Gran-Gran, think of Sokka's Master, the renegade firebending master from the first season... And then imagine Sokka, Toph, Ty Lee 70 years older, in their place. If Legend of Korra was 100 years to the future, those kind of links would be much harder to make. I think and hope it was intentional, and that there will be all kinds of tiny hints at all the things that happened in the first series.