Microsoft"can you build a "site template" based of a current site that has been customized, including the web parts, pages, and even content?"
As I mentioned above, that's what we've been doing. We layout/design a site with dummy content for various usages/channels and then create a template to speed up deployment and "encourage" compliance with company standards. The process for modern sites is a little clunky, but it's way more useful and flexible than the "site in a box" templates touted here.
There's a how to guide here: http://webbrewers.com/modern-sharepoint-create-a-site-template-yes-communications-sites-too/
So by classifying types of sites, and using templates we built, we're already doing what Msft's big announcement is offering here. And I never used site designs as they seemed way too limited in terms of customization options.....
