Don't get me wrong, I can see the need for a new 50mm f1.4 replacement in the lineup, but I don't see it as all that necessary compared to the one big hole in Canon's arsenal.
I'm speaking of course, of a relatively small, lightweight, good quality, flare resistant approx 20mm prime lens. Canon simply doesn't have an answer to Nikon's new 20mm f1.8 G.
I already own the 24-70L mark I for years, but am transitioning to primes for pleasure. My preferred lengths are 20,35 and 85. The crops available from high MP cameras mean you can crop pictures from a 35mm to become a 50mm equivalent, 50-85, from an 85 to a 135 etc, without the need to carry a big zoom, and with the advantage that the fast aperture prime will have better depth of field control than a zoom while still having adequate resolution even after cropping.
The Canon 20mm usm f2.8 is a great lens when used well, but there's no getting round its flare.
I really hope Canon will introduce a 20mm (or 16-19mm) prime, preferably with IS. I would take this in preference to the 16-35 f4 IS since it would fit in my pocket, probably have a wider aperture, and make sure I frame the ultra wide photo properly rather than use zoom badly - which I am prone to until I get more experience with ultrawide composition. A 20mm has a notably different aspect to a 24mm or 14mm therefore it is a big gap for Canon. I can't yet justify the 14mm f2.8 or 17mm TSE as a beginner ultrawide photographer (the 17 SE is pretty big too), and the 17-40L used is almost the same price as the 20mm usm used. It therefore feels wrong to put money down on the old 20mm which has inferior image (in many situations) and build quality.
So here's hoping for a new Canon 20mm f2.8 usm is.