Often small changes make the difference, so when I fine-tune a mix which sounds already pretty good, I usually write down the values of faders/knobs I change to have a chance to return to the initial position when the change did not make it any better. Or I enable automation and draw the initial value on the line. Both is time-consuming and kind of complicated. A possibility to add a coloured line to the actual fader position in the mixer (f.e. with a command in the context menu) would really make sense. Think also about writing automation moves, where you want to come back to the initial position etc.