Makes me wonder: could "serif to sans-serif" be an OpenType variable-font axis — i.e. could a font be designed to gradually "grow" serifs out of a sans-serif base, and to look good at any "degree" of serif-ication, the way modern variable fonts are designed to look good at any "degree" of weight?
What would a 30% serif'ed font even look like? Would it have little vestigial bumps only visible at large optical sizes?