Blend Modes
It would be cool if you could draw_from and specify a blend mode like multiply or overlay. (Like with layers in Photoshop)
This could be used to create some nice 2D lighting effects and other effects.
getPixel() - This may have already been mentioned...
It would allow for greatly enhanced AI systems and other effects if it were possible to get the pixel color value of an actor or at a screen coord. The ability to choose which would be awesome.
getPixel(<taget actor>, <x coord>, <y coord>)
or smoething like that...