It appears as a small but heavy package, this lens design chock full of aberration correcting glass element groups. First noticed by Canon News, two variants have been patented: one as a 15-35 that appears a bit big for an f/4 wide angle; and a 16-35 that looks properly compact.

The existing RF 15-35 f/2.8 is probably the best lens in that focal length range and aperture yet made, easily besting the third party lenses that tried. This patent for the f/4 variant is clearly a downsized version of the same design. The f/2.8 version has two ultra-low dispersion elements and three molded glass aspherical elements; complicated stuff to manufacture. It is likely this new design would require those expensive materials to work, as otherwise the design would have had to change more radically.

If Canon does release a lens with one of these new designs, it should be a corker, but not cheap. The new design:

… and the RF 15-35 f/2.8 design: