Progress indication and scrollability indication. I'd actually like to see scrollbars enhanced rather than diminished. So for example I would love to see indicators of headings or breaks or page bookmarks. The latter could even be clickable.
I agree! A lot of code editors are using them in a nice way to display version control info or errors/warnings. Chrome is using it to a small extent when searching using Ctrl-F, showing where in the page the results are. In both cases it's super effective. But they are definitely still underused.
