I think if you added a few more Volume plugins, say Volume-1, Volume-2, Volume-3, etc., that would take care of the need for input and output gain knobs on every non-linear plugin. Since it's just a cut and paste of existing code, it's a lot less work than modifying all the distortion type plugins. It also avoids having to allow more than one instance of a plugin. You only need a few more, since most effects chains won't have too many distortion plugins and many of them have either an input or output gain knob now.
I think if you added a few more Volume plugins, say Volume-1, Volume-2, Volume-3, etc., that would take care of the need for input and output gain knobs on every non-linear plugin. Since it's just a cut and paste of existing code, it's a lot less work than modifying all the distortion type plugins. It also avoids having to allow more than one instance of a plugin. You only need a few more, since most effects chains won't have too many distortion plugins and many of them have either an input or output gain knob now.