So I see some of you good folks putting forward the idea that next season will be about the avatar realising that she isn't needed anymore, that the world has grown away from needing her.
While that is an interesting theme, I have to ask: How?
There's still much conflict in the world, with the equalist rebellion in Republic City, the two water tribes fighting each other and now the Earth Kingdom being reduced too anarchy, so the world simply going all hippie and peacelike and thus not needing any higher authority to judge them can't be the reason.
New technology making bending - and in turn the avatar - useless? I can't think of any that has been revealed in the series. Also, much of the technology through the Avatar series have in many cases been tied to bending (one example being Korra's firebending-ex producing power for the city through his lightning-bending), which is not strange since bending is kinda awesome. I mean, full controll over an element in nature? That alone is awesome and I have a hard time seeing how that would ever turn into a useless trait.
And the Avatar is able to bend ALL elements.
Korra getting her butt kicked one too many times (even though she was supposed to be good at fighting while having trouble with the spiritual side of bending..)? While that does provide a dramatic clue to the whole "Avatar-going-the-way-of-the-dodo-bird", it doesn't do much than paint her as an incompetent Avatar. That doesn't necceserilly say anything about the Avatar's role in the big picture of things. Surely there have been one or more clueless Avatars before, seeing they have been reincarnated a couple of thousand times.
Just my thoughts, now I like to hear yours.