^But Republic City is a microcosm for the new world. It's a city where people from all four nations live together in a unified society, just as the whole world does in this new era. As it goes, so goes the entire new global order.
After all, this is a new, industrialized world. In the 70 years since Sozin's Comet, the world has modernized and been brought closer together by radio, automobiles, and other industrial-era technologies. So the world of A:TLA, where the various nations and cultures were more separate, is giving way to one that's growing more interconnected. Korra
is about events and changes that affect the whole world; it's just that we're seeing them happen in the place that serves as a social, political, and economic focus for the world as a whole.
Here's a nice new interview with Konietzko and DiMartino that gives some new insights: