Page template picker / dummy content

I’m building a theme, aimed at novice users, and one feature I’ve built is a page template picker. Before the New Post or New Page screen, there’s a custom screen where the user gets a drop down of all the page templates, post formats, custom posts, etc. For instance, the user can select New > Photo Gallery > No sidebar, large post image. The flow is visual and currently shows large previews that are just static images. My question:

How would you go about rendering these previews live? And with the current theme styling? Is there a way I can stash ‘dummy’ content somewhere and render views on the fly? My first idea was just to create a dummy site somewhere, with one of each template/content type as a sample I could load in, but I’d really like whatever changes the specific site has in place to be seen in the preview.

This is sort of a high-level question without a specific answer, but feel free to spit out whatever ideas enter your head. I’ll look at anything.