I’m going to suggest a simple enhancement of the local adjustment tools. You are welcome to suggest ways that the feature I’m suggesting can be accomplished using currently existing tools–the best enhancement is one that is already implemented!
My enhancement is to add the option to insert one chroma/luma selection point in any mask that doesn’t already have chroma/luma support: the brush, auto mask, and gradient filter masks.
If no chroma/luma point is added, the mask would continue to work as it always has. When a chroma/luma point is added, the selection is adjusted in the same way as with control points/lines. The chroma/luma point would not have to be within the mask. It could be moved or deleted.
The enhancement would be most useful with the brush mask. With a control point, you get a radial mask; with the brush, the mask can have any shape.
The graduated filter and the control line would become very similar, but you can combine the eraser and brush with the graduated filter and not with a control line. Actually, the only reasons for keeping the control point/line would be for backwards compatibility.
Control lines and control points can be combined in a single layer in complex ways (including adding protected areas), but this can get confusing. Each control point/line selects something different, but the same adjustment (even the chroma/luma selection) is applied to everything. The control line is a bit better in this respect since the sampling pipette can be moved independently of the gradient. With control points, the sampled point and the center of the radial mask are the same (if there is a way to move these independently, let me know!).
The enhanced masks are more straightforward: there is just one chroma/luma point. If you want to include/exclude something, just use the brush and eraser as needed.
With this change I could select two separate regions of my photo using the brush, pick a specific single chroma/luma point, adjust the chroma and luma as needed, say, to mask just the shadows and then apply an adjustment (sharpness, for example) to the masked pixels. This simple task is tricky to accomplish using the existing control point/line: the basic mask might cover too much or too little and with multiple control points, you might not be selecting the same shadow region in each.
This is not how I actually want masking to work; it is just the simplest change I can come up with that gives me 95% of the ideal solution.
With the ideal solution, rather than a chroma/luma point selected on the image, I could specify independent mappings for hue, lightness, and saturation (or just use a picker to reproduce the current behavior). For example, if I wanted to select “shadows”, I could define exactly what ranges constitute shadows and how to taper the selection as I approach “black” on one end and “midtones” on the other. The current chroma/luma sliders provide some control, but not complete control. Still, I could live with the sliders, and I could live with just chroma/luma (as opposed to HLS).