Looks like I'm not the only person to have this idea:
https://medium.com/@hrescak/exploring-dynamic-layout-in-sketch-fdf0e825d1cf
I really like the visual prototyping (near the bottom):
But I still feel like there's probably a more fluid way to move from "I'm just starting with a design and don't know what it should look like" to "okay I basically have the design elements and know what things should be structured components".