Canon makes the 8-15mm already, so to be the widest, it must be really wide ...
Widest FF zoom lens: 12-24 L f/2.8 USMExcept 12mm is already being done; they'd have to make it an 11-xx lens to be the widest.
IMHO, a wider than 180deg circular fisheye is very unlikely - it would have the granddaddy of niche market, and compete with Canon's own 8-15mm fisheye zoom.
So I'm for an ultra wide lens, as
I. Both Nikon & Sigma have wider zooms, so there's a market for such a lens.
II. Canon camera owners buy Nikon's ultra wide zoom with an adapter, so Canon is losing sales, and specifically the kind which might make people switch brands. Again, more so than to the Nikon 6mm.
III. Nikon issued a patent for a full frame 10mm prime. It wouldn't necessarily get into production, but it helps giving Nikon an image of the leader in ultra wide lenses.
So I'm for an ultra wide rectilinear zoom.