Similar in idea to the Gamut masking color wheel, it could be very handy to control the color picker in the dimensions of H, C and Y independently. Currently picking a color "picks" all the HCY values from the canvas. If you could select H,C,Y independently, you could have a more organically masked color gamut and palette derived from your in-progress painting-- basically a way to help achieve a limited and cohesive palette without planning ahead so much.
One option would be to have a tool option panel for the color picker that has three checkboxes to select Hue, Chroma, and/or Luma. For example, if you check Chroma and Luma(Y), then any color you pick from the canvas will only adjust the Chroma and Luma of your current color to match the canvas color's C and Y (as much as possible). Likewise, checking only the Hue box would allow you to pick the Hue from the canvas and preserve the current Chroma and Luma of whatever color you currently painting with.
Another option is to create new key bindings-- Pick Hue, Pick Chroma, and Pick Luma. Then you could just map those to keys and if you wanted to pick just the Hue from the canvas maybe you'd press Shift+R, Chroma might be Ctrl-R, and Luma might be Alt+R, and just R would pick HC and Y as it currently does.