Here lets discuss what the future holds for the avatar world.
We know that science and technology are progressing at a monumental rate in the avatar universe. In Katara's laifetime itself (about 80-90 years), we have seen technological progress go at a blistering pace catching with our early 20th century along with an few really advanced concepts such as tazers, submarines and advanced airforces. Its only a matter of time before computers are invented (perhaps the simple ones are already there), space exploration is started and so on and so forth.
One more thing, in the last episode of season 2 of the Legend Of Korra, her connection to her past lives is lost as Raava is taking hit after hit and growing weaker. Therefore we can assume that those memories and experiances are available to korra indirectly through Raava, when Raava is reborn.
We know that once computers start evolving, they do so at a frightening pace. It may not be long before in Korra's world that people start wondering about self aware computers. This may happen in Korra's lifetime itself. Also, in the last episode of season 2, she seems to have killed Vaatu, who inexorably will be born from Raava, the spirit part of the avatar. I have a feeling that darkness in light will be introduced by Asami in Korra's life, presumably over Mako.
Coming back to the point, when (not if) computers become self aware, what will the conditions be? As they were in the Terminator universe; humanity caught off guard? As in the Matrix universe; pitched battle? Or as in i, Robot universe; self aware individual machines with free will and without a central dictatorial entity controlling them?
This is a tremendous avenue for the story line to evolve into. consider this: a few avatars down the line, computers become self aware. There is conflict and as always, people look to the avatar to restore balance. But the avatar is simply not equipped to deal with this. They are not angry spirits, but cold, logical computers. Who will rise to the occasion? Its an open question, but my favorite setting is as follows:
Cybernetics has evolved to the point where prosthetics can be like actual body parts (i, Robot universe). Someone important who is so badly injured somehow (including head injuries) needs extensive prosthetics and to restore his/her mental and physical faculties, which is absolutely necessary at any cost. A radical new procedure is applied: parts of his/her brain are replaced by machines- CPUs, memory registers, etc making that person a true hybrid, a real cyborg, the first and (upto that point) only one of its kind. Later on, conflict emerges between humans and spirits on the one side and machines on the other. The avatar takes the human-spirit side which doesnt turn out to be a wise decision and only makes matters worse. Disenchanted by the avatar, hackers take matters into their own hands and a pitched cyber war erupts. Then this person (who combines the personalities of Neo from the Matrix and Sonny from i, Robot) steps in.
I leave the middle part to your imagination, but in the end, a new era begins. The age of two avatars one who is part human, part spirit and kepps balance between humans and spirits and one who is part human, part machine keeps balance between computers and humans.
These days a franchise is considered to have come of age when it starts introducing alternative timelines, other universes and the concept of multiverse. with space travel, all of these will come about in the avatar universe and it will probably become a multivers. Questions:
Which other multiverses will it intersect with?
What will the avatar do in the space age? Fight baddies in space? How?
Korra managed to be relevant in the transition from crude semi middle ages type of world of Aang to early 20th Century. How relevant will the avatar and the spirits be in the space age?
The scope of these possibilities is mind boggling. The best days of the franchise are certainly not behind it.