Just keeping up with Photoshop.
Hold down Alt key when dragging nodes to see clipping on image (as already exists in Levels).
Show vertical and horizontal lines from selected node (or maybe just while moving) help align with axes.
Show percentage X and Y values for selected node (and allow input this way).
Have an 'auto levels' button to stretch (and unstretch) the histogram to remove dead space at ends.
Allow presets to be loaded here.
When using picker and when moving mouse around image, show point on curve with moving node to reflect lightness of underlying pixels at mouse.
Thanking ye kindly sirrah/modom!