I had an idea a while ago that it would be nice if there was an option to somehow have a visual indication (feedback) of level-like/threshold-like plugin parameter settings against the track waveform in the Editor itself. The more I think of it, the more I like it, so here it is.
Let me give you an example of what I think. Say I have a Limiter plugin with a threshold parameter that I want to set so it is not too high (or it will not perform any limiting) and not too low (or the signal gets distorted) but is just right, gets to work at the peaks but does not touch the bulk of the audio. Of course you can set it using your ears, and looking at the post-FX level indicators. But I imagined that in Ardour (eg. by holding Ctrl or some other key while adjusting this parameter in the plugin GUI) there could be drawn a pair of thin red horizontal lines in the track itself that corresponded to the currently set level. As I adjusted the threshold parameter, this line - drawn against the audio waveform - would show exactly where this threshold is set in relation to the actual audio signal.