Canon has super lenses at the telephoto end, could they do crazy things with UWA too?
This lens would have a 152mm diameter and cost over $7,000 at launch assuming the same optical design as Nikon's was used and Canon's standard mark-up on the cost of ultra large lens elements was used.
In short, there is no way this would ever make it to market.
Canon could sell 10 times as many f/2.8 UWA lenses as they could f/1.8 ones.
In fact to make up the smaller market for the lens Canon would have to charge close to $17,000 for this lens, if not more.