I would personally like to see a CSS framework that has absolutely
zero opinions about design. Meaning, radio buttons are actually the standard html5 radio buttons, but perhaps with titles handled in a unified way that works across all browsers. Form elements are given proper layout CSS that solves a lot of common issues across all browsers but no additional styling is added. Etc. I don't want my site to look like yet another bootstrap site (YABS) and overriding all of bootstrap/foundations/whatever's styles is a huge PITA. I usually just end up rolling everything from scratch for sites where I want full control over the design.
edit: I'll post this as an edit since there are multiple responses referring to http://www.getskeleton.com. Skeleton is similar to what I'm asking for but it seems to have stopped short. I would love to see unstyled carousels, alerts, modals, sticky headers, images with inline comments, etc. Basically, all the really nice stuff you get in Bootstrap and Foundation but without any of the styling.