Color spray or color dynamics
I would love the ability to vary colour when painting with a brush (or just value if painting in a grayscale channel). See the "Colour Dynamics" tab in photoshop or "Color Spray" zbrush as an example.
I really feel this feature is absolutely necessary, as I'm sure anyone who has ever textured in Photoshop or Zbrush would tell you, this is used all the time.
Controls to vary the hue, saturation, brightness etc when painting would be a great feature and really improve the painting ability of this program. Painter is good with masks and filters but I really feel the actual painting options are lacking at the moment. Could also add to this the ability to vary opacity then would really be in business.