Instead of having a Desaturate filter, a Lighten filter, a Darken filter, and a Hue filter, my idea is to add one filter that has them all in one. So it would look something like this:
Hue: 0-360 Slider (defaults to 0)
Saturation: 0-100 Slider (defaults to 50, 100 is fully saturated and 0 is fully desaturated)
Lightness: 0-100 Slider (defaults to 50, 100, is max brightness, and 0 is min brightness)
I think this could be very useful for a lot of people, since right now you can only use one filter at a time, and the Desaturate filter can't increase saturation.