A Canon patent from October, 2017 resurfaced on the Canon rumor mill today, and it describes something pretty innovative from the Japanese camera giant. The patent shows a lens adapter—ostensibly designed for a mirrorless camera—that features a built-in variable-voltage ND filter.
The ND filter component is "formed from an electrochromic device [or a liquid crystal element] which changes light transmittance on application of a voltage." That way, you could electronically and silently change the amount of light hitting your camera's sensor without stopping down the lens.
As Canon News points out, this would be particularly useful for video shooting, since it's silent, and it would allow you to separate depth-of-field control from brightness (i.e. shoot with a shallow depth-of-field in bright light).
We could see this technology more readily appearing in a Cinema EOS system, but Canon has (naturally) written the patent as broadly as possible so that no one else can use the idea for stills. Canon Rumors also speculates that it might be the patent behind a rumored EF/EF-S to EF-M adapter specifically designed to adapt cinema lenses to an EF-M mount camera.
As always with patents, it's impossible to know what parts of this idea—if any—Canon plans to bring to market. But it's a rather innovative design that we wouldn't mind seeing in action.