Well, the creators have said they're keeping this series based in Republic City to differentiate it from the globetrotting A:TLA. But I agree, it would be nice to learn something about how the rest of the world has changed. Hopefully we'll get occasional field trips, or learn about those places from visitors or news reports. Maybe if the show gets another season beyond the slated 26 episodes, they'll open it up more. Or maybe there will be tie-in comics.
On that subject, there's a canonical comic coming out called The Promise
, which is set a year after "Sozin's Comet" and serves as a bridge between A:TLA and TLoK. It's in three 80-page volumes, of which the first is already out and the second is due in June. I'm eager to get my hands on it. I'm hoping it will reveal that "incredible story" of Zuko's search for his mother.