I think it would be extremely useful to allow plugin developers to add text before the line numbers. This would allow for plugins to place git blame text for each line, or profilers to put statistics on that line (like AQTime). It would even allow for plugins to add short note messages for the lines as well.

At the same time it should be made possible to have more regions in gutter area per line, so that one could have both a bookmark region, and a breakpoint region, if not more.