Surely this has been mentioned before, but I did a search and found nothing, so...
As an Illustrator, I would really like to be able to set a vector path and then stroke it with one of the drawing tools [i]using the pressure sensitivity[/i] of the Wacom pen/tablet. Painter has this, but I prefer the way Photoshop handles a lot of other things (layer management, vector shape transformations, etc.).
This would work similar to the Magnetic Lasso so that I could follow a vector path with a Brush tool, varying pressure during the stroke. I am aware of Ps's current ability to stroke a path, and the "simulate pressure" part, but neither of these give me the control I'm looking for.
An example would be to draw an ellipse with the Vector Shape tool, then stroke a portion of it with the Brush tool, varying the size and/or opacity along the way. Move or transform the vector shape, and stroke again.