It would be really cool if we could have a feature like the engraving tool in Inkscape.
For those of you not familiar with inkscape, the engraving tool simply makes the width of a path dependent on the brightness of the underlying artwork.
What could be even more cool was if it could be made so that it could be done with already existing lines. This would make it possible to apply the effect to expanded blends and thus get the effect over a wide area of evenly spaced paths.
I think an engraving feature would be really useful to a lot of people