I love that idea!
The designer guidelines that the canonical people made is for me a good balance between density of text/buttons, space and usability.
For me in todays fancy new designs it just needs to look good. Developers have many good technical ideas, but lack the experience in designing something that looks good/consistent (if it works, why change it?).
For me the point to make many mockups and to discuss them and adjust them accordingly to feedback and use it as a base for everything would be necessary and would help all those other unity8/7 spin-off.
But I think in order to make it possible designers must help developers create these mockups. or create a lego like framework in gimp ?
One of the ideas I have is to create a unity8/7 design group that defines the future/base of the unity like Distributions and bundle all of these different/convergence?! goals ?