Upload & Sell: On
The wider the lens, and the larger the aperture, the harder it is to make the front fairly flat so that screw on filters are practical. The Canon 17mm TS-E has the image quality and low distortion that we all want, but has the protruding front, just like the Nikon 14-24 does. But Canon could make a filter holder for square and rectangular slide in filters that bayonets on like the lens cap does, and add that feature to help sell any new UWA zoom.
I personally am not using graduated neutral density filters as much as I used to, and find that doing image combines in post processing actually gives better results. That has me less worried about mounting a filter on a UWA zoom. I actually would like to see an option for a larger hood that could be mounted on the lens via the bayonet, to give better flare protection, as well as act as a better protector of the front element when the lens is in use. I really fail to see why we do not all ready have such a supplemental lens hood available for all the Canon lenses that have bulbous fronts. Such hoods would be on the large side, but would be well worth the hassle they would present. They would not need to store reversed, because you would need the bayonet for the dedicated special lens cap that covers the bulbous element.