I've got an introductory adventure sitting up how the PCs all escape, which has a built-in plot hook that we can optionally explore later in the game.
Re: The Gym Leaders, whether we need to flesh them out or not really depends on how the PCs go.
PoBURI controls the region, but a good chunk of that control comes from making themselves look very good to the public so there's a bit of leeway in what exactly we do after the intro.