My comments on demo content and previewing a theme (other than the proposed theme review process on w.org theme directory) as follows:

As long as we think a theme is a Blog Theme (only), a curated demo data and Customizer Preview is the great addition and well enough.

And then there’s the concept "WordPress as a Site/CMS"- niche theme needs different data-sets, along with default settings and those who want to practice the best- needs (free) plugins enabled to display the Theme’s full potential (i.e Post Formats, Extended Features like testimonial, portfolio etc).

I think:

- Current Author Lin/Theme Homepage Link points serve as ‘Introduction of the theme and Documentation’ (The Doc)
- Allow authors to include their hosted demo link (The Demo)

That will open the possibility to build-up a ‘very interesting’ or ‘out of the box’ theme, and make it to theme directory. Yes, it’ll boost up-sell, too.

What I see is the opportunity to:

- Take a peek in org demo and get the idea of settings/play with settings
- Take a peek in author hosted demo and taste it’s optimum view/features

I would suggest to at least open door for author hosted demo for a while (3-6 months) and see how bad it goes, and/or track how many users clicks on org/hosted demo links.