At this point, I'm using SASS pretty much exclusively for nesting (and where required to customize other styling systems-- mostly older Bootstrap). Once I can assume CSS nesting is everywhere I need it, I can probably eliminate SASS in most places where I'm using it.
(I'd also like more/better color functions out of vanilla CSS. But that's not necessarily a dealbreaker for me in most cases.)
I would recommend using PostCSS. It allows you to compile future CSS for browsers today. When you feel comfortable with browser support you can just remove the processing step
(I'd also like more/better color functions out of vanilla CSS. But that's not necessarily a dealbreaker for me in most cases.)