The work being done by Bojhan and others in this group, as well as within the Spark and other initiatives is simply amazing. It will undoubtedly lead to better UX and usability for especially those that will work with content.
As a site builder I am very eager to be able to get more involved in this work. Personally I am most interested in helping to test the usability and UX when constructing these editing interfaces, and provide feedback on that. Especially since it is those interfaces I as a site builder will be using the most in my work.
However, this is not always so easy. For Drupal 7 for example, its wasn't really until alpha 4 or 5 that I was really able to install and get it working. By then core where already delayed and developers felt more pressure to get it ready, leading to that it was very difficult to get any improvements to the site building experience done. There simply wasn't much time to change things...
I think it would be very helpful if it was possible for us site builders to be able to get involved earlier in testing. Especially as it is in most cases not until you try and configure something that you also will discover limitations, bottlenecks and other things that makes constructing (UI configuring) features too complicated.
This doesn't necessarily have to be working site builder interfaces, although that of course would be optimal. But just being able to see mock-ups/prototypes of the interfaces we will use would help us to at least better visualize how things will look.
Personally I would love it if this also could lead to a site builder usability team that focuses on the needs and challenges site builders are faced with when configuring site features in the UI. I am convinced that would be a great addition to the work already being done to make Drupal easier and better to us, and I would for sure be the first one to sign up to help out with that.