I don't know if some of these things have been mentioned before or not, but...
1) Layer tinting. I know this can be done with masking and transfer modes, probably, but having a tint control would be that much simpler.
2) Procedural textures. To break up the flat colors of a vector shape. These would be superior to using image layers because they would preserve the advantages of vector shapes, i.e. they wouldn't lose detail as they got closer to or farther from the viewer.
I would like to use tinting option or color balancing (animatable).
This option can be very good for sunrise scenes, lightnings scenes, etc.
It`s also can be very helpful when the client ask you: "Can you make it more blue?"