So, you know how in Avatar, Katara would sometimes dress up like a spirit or a ninja and turn into this dark, relentless, unstoppable badass? She was, basically, a teenage girl version of Batman.
So now we've got a new series, set nearly a century into the future, centered around a dark and corrupt city infested with crime and dangerous zealots. And we've got a new teenage girl, who's also a waterbender. And while we know Aang's gone, we don't know about Katara yet.
All I'm saying is, if Katara shows up as a grizzled old recluse that has to train Korra to take up her mantle as the city's protector [http://tvtropes.org/pmwiki/pmwiki.php/Main/BatmanBeyond], that will be awesome.
Bonus points of there's a gang of sociopathic Azula wannabes.