Sanderson has this tendency to completely change the world in ways that you’d never think could possibly work… and yet pull them off. He did it between the first and second Mistborn books and he just did it again.
Worldbuilding-wise, so many things have changed. Yet he hasn’t really broken the rules that he set down in the previous books, just expanded them, exploring areas that we’ve never seen before. We learn about people that live far from the Lord Ruler’s rule. We learn about new and fascinating ways that the Metallic Arts can be used–Feruchemy in particular. And we start to see just how magic and technology can be combined.