I think the concept of having Korra re-learn the elements would just be a rehash of The Last Airbender, except more boring since there isn't a war going on at the moment. I would much rather the writers explore new ground than just repeat the same formula.

And yeah, Nickelodeon didn't believe in Korra at first. They thought the show would bomb because it had a female lead. It wasn't until it was proven successful that they agreed to another season. That's why the ending seemed so...'tidy.'