Several years after the series has "ended", the changlings get over trying to conquer equestria and industrialize. They start out initially well, but then they squander the resources, and pollute the planet.

Some nations secede and wage war too, which makes the planet near-uninhabitable.

Pony-kind must now reach a new planet, settle it, and re-start their civilization. Simple, right? Not so much, as they will soon find out...