I agree this is a great idea.
A true “Levels” tool that can be used to recover highlights would especially be useful. (For instance if I lower the output range from 255 to 240, instead of clipping the output at 240, you should bring the highlights in the source that are above 255 down into the 240-255 range.
A levels tool like this really will help to make contrast and brightness adjustments easily with one tool.