Here we go!
This is the 'princess' patch, which is the entirety of SCOTCH that we've been working on in the 'princess' branch (in this sandbox). We don't have many tests written, and we haven't actually swapped in the use of our display objects for any of the normal Drupal's normal routes - what we have is a test route, which you can see at /booze, that renders through the new method. There's no cheating there - hook_page_build() and hook_page_alter() are never called. You only get to make it onto the page if you are a correctly-participating block. There's also no UI for the new block system. The existing block UI remains in place, so to change things, you have to edit core/profiles/standard/config/display.bound.frontend.yml - that's the display that's used to render the /booze route.
For those concerned about my 'irreverent' naming habits, the only place that "booze" now appears in this code is on this test route, which obviously has no purpose other than being a development-in-progress demo, and thus is not destined for core anyway.
There are lots of new concepts here, and rather than delay this patch further by writing a novel trying to explain everything initially, I'm just gonna leave it at "go poke at it, have fun" for now. We can shape up this issue summary more over the coming days.
Anyone with a better idea for an issue title (or the version) should feel free to change it.
Easiest way for folks to check this out, at least to see the output, would be to pop it over to simpletest.me and go to /booze.
Here is the link to test the latest code in the princess branch Test the princess branch on simplytest.me