We've spoken about the existence of an EF 14-24 f/2.8L in the past, a lens that a lot of people want to see come to fruition.
We've been told that an EF 12-24 f/2.8L exists in prototype form and could become a product instead of the EF 14-24 f/2.8L. I can only recall Sigma making such a lens for full frame cameras. Going this wide would certainly put some space between the cheaper (assuming based on Canon pricing history) EF 17-40 f/4L and EF 16-35 f/2.8L II.
This is the first I have heard of an EF 12-24 f/2.8L, so take it with a big grain of salt. This could be a better range in an f/4 variant, as it would be great for both full frame and APS-C and would really reduce size and weight compared to a 2.8 version.