A gutter along the side of the text which is modifiable. Colors, buffer, size, etc. This would be very useful for things such as line numbers (relative or absolute), colors for denoting when a line has been modified and not saved yet, an editable buffer for marking a line of code as you please, etc. This would be extremely useful for the way I'll be using 4coder (and other people as well, I hope).
If you have any tips and tricks for how to accomplish some of this in the meantime, that would be great. I've been trying to mark lines in other ways and haven't come up with a good solution yet.
Cheers