Helen Hou-Sandí started a conversation in WordPress Trac that I'd love to see get more participants. It involves the role “dummy content” — or demo content that would appear during live previews in the customizer — could play to give users an idea of what would look best in a given area of a theme.
I think it would be a great help to WordPress users, as well as for theme authors looking to show off their themes capabilities. for instance, what if the customizer could render a
demo.json file, defined by the theme, with a fallback to default demo data? And perhaps plugins could hook into such systems as well. I think there's a lot of potential with this.