Some news from Comic-Con:
As mentioned , the cabbage merchant will not appear, but "his legacy will be present in some form."
As an evolution of the first series, The Legend of Korra will have more sophistication to it, but still keep the humor, fun hybrid animals, and action.
It won't cover more mature subject material necessarily, just different mature subjects. "It won't be another war, but a different sort of conflict."
On whether or not we'd see a love interest for Korra: "We love that stuff. We're total cornballs, we love cheesy teen romance. Rest assured, there's plenty or romance happening in the story, but it's the same balance of ingredients that we like."
Katara IS in fact the mother of Aang's offspring, who will be central to the show. "We didn't mean for it to be a big super-secret, that they end up together."
On the persecution of Airbenders being compared to the X-Men registration act: "We'll see something like that, but in sort of a different structure."
Will it be a very different world from where we left off? "Yes and no, the world has definitely changed, and evolved, and advanced, but we're very conscious of keeping the same feeling. Not totally different, but it's definitely generations later."
"There will definitely only be one Avatar at a time, and it is Korra."
So will Aang have any kind of presence? "Yes...in his way."
DiMartino and Konietzko will continue to work with Joaquim Dos Santos and Ryuki Hung, working to improve the design and animation.
The Legend of Korra will in fact be a mini-series, with Michael and Bryan directly writing each episode.
"As a mini-series, it's going to be really tight. While we really love the filler episodes, those are some of our favorites, this thing is just lean and mean. It's a really cool driving story."
http://www.ugo.com/tv/comic-con-2010-legend-of-korra-interview