Maybe I've missed something, but so far we haven't seen anything wider than 16mm and an aperture of at least f/2.8 with an actual filter thread. If I saw it right, all three RF wide-angle lens design patents showed a bulbous front element. Also, maybe the zoom range has be narrower, like 14-28mm f/2.8 or 15-30mm f/2.8, that sounds much more realistic.The original 16-35/2.8L takes 77 mm filters, and one of the characteristics Canon has talked about for wide/normal RF lenses is smaller front elements and larger rear elements. This is because they are not so strongly retrofocal so they don't need such extreme negative elements in the front group which are large and inclined to be bulbous in a DSLR lens.
So maybe an RF 14-35/2.8L could have a 77 mm filter thread?
Or it has to be reduced to f/4 like with the Nikon.