Instead of "Themes" I would prefer things to be categorized by "Features". Menu features, Category Features, Tags Features. Image Features. Then let me pick which ones I want! Each feature would have short codes, partials, html, whatever..
Basically, factor the themes into their differences, and sort those differences into different categories, and Git that all together into something that can be unpacked into MY custom theme. Kind of like hugo does for sites, but break down the logical parts of themes!
3rd beer, hope this makes sense..