The filter I would like to see is a GND that has the cut-off at 1/3 of the field rather than down the middle. I would also like this filter to rotate (like a polarizer) so I could place the 1/3 ND field anywhere in the frame - (top, bottom, left or right). That way, I would not have to switch to a Cokin type setup to effectively use a GND filter.
I hate the standard 50% cut-off on round GND filters because it forces the photographer to stick the horizon in the middle of the image - boring, boring, boring!
Most of the time, I don't even use the GND anymore.

