I like the idea of it being non-linear. In terms of features, being able to paste an image (e.g. like pasting into a github comment, or into a gdoc) would be great. I find I use screenshots into the clipboard more than saving to a file, however I don't know if others do as much nor if Windows / Mac supports that.
Mac: Ctrl+Cmd+Shift+4 to grab a rectangular selection to the clipboard. Does windows have something similar? I've been using PrintScrn and Paint but it's comparably very awkward. I know there are apps, but "working in places where I haven't / can't install apps" is a requirement.
Mac: CMD+SHIFT+4 (Ctrl not req'd.)