Allow plugins to display things in their own gutters

I imagine plugin which marks code complexity, non tested code, toggling breakpoints etc.

Yes, I think there are all sorts of use-cases here...  Having additional information available with — for example — a swipe to the right (to the "left" of the line numbers, so to say) could be very helpful.  I often check "blame" to figure out why some particular code was introduced and what might depend on it, but test coverage info is also a great idea.

Never thought of such a feature before, but really this could come in handy. A special gutter with a lightweight API so that plugins could display information gathered by running a command in the background… pretty neat?