I just got Art Rage 2 and its great. I'm definitely getting Art Rage 3 when it comes out.
Altho I've only used AR for a short time, here are my suggestions:
Simple things like---copy, paste and delete things onto or from layers.
Being able to make selections and delete selections from one layer to another.
How about a preview window when you are opening an image. It makes it easier to find the picture you want instead of just having a list of files to chose from.
It would be nice to have shapes and lines with strokes and fills.
How about simple filters, such as blur and sharpen.
I agree with others who suggested watercolor brush and being able to make custom brushes from images, or import them from another program.