As a boilerplate it's already great. Don't touch it. It doesn't need to become a framework, it's the alternative to a framework.
I'm one of the people who do what you wanted in the first place and edit all the code by hand inside the theme, not in a child theme. As far as I understand it that's the idea, and It's a good one. I learned everything I know about building themes from ripping BlankSlate to bits and writing custom CSS for it.
I now have a standard theme framework based on BlankSlate with the addition of a slightly modded style.css file that has a few standard bits in. A nice easy to mod button, some basic form styling and a couple of other bits I would forget to style if they weren't there. That's enough for me. If I want jQuery I'll add it myself, same with Google fonts (et al).
I use BlankSlate to get away from framework bloat. If you want to improve it keep it simple. Streamline code by all means if it makes it better but please don't mess around too much.
I'll keep using it for all my jobs anyway.
All the best.