One thing I'd really like to see is more powerful way to "grade" a picture than what we have today, more in line with what is available in video processing tools.
Today we basically have Temperature and Tint as global effects, and if we want to grade Shadow and Highlights separately, we can go to "DuoTone", or use the brush to alter part of the image.
Most video tools, Premiere first among them (no pun intended), allow you to grade separately shadows, mid-tones and highlights. Pro video editors can't live without that feature - it defines the mood, feeling and identity of a sequence (watch carefully any modern tv serie to get convinced!).
So, I dream of a drop down next to the two color balance sliders that would read "Entire image/Shadows/Midtones/Highlights", where the boundary between the three tones are the ones defined in the curves module.