There are various places where you could implement this, and various extents to which you could choose a more or less robust implementation.
The feature that compelled me to ask for this feature would simply be a "saturation" option for the Curves adjustment.
Having additional options to effect hue and brightness in the same way would be nice too, but not as important as saturation for me.
Adjustments for HSB would also be perfectly well suited to being added to the "levels" adjustment, and one could envision a version of this idea where "curves" manipulators could optionally replace the sliders in the Hue Saturation adjustment.
My preference would be to have HSB options for the Curves adjustment, but any of the above solutions would be great.