It seems like 40mm (on FF) is about the sweet spot for pancakes because it provides a very natural perspective and is a simple lens requiring few corrections to achieve decent optical performance. My favorite lens on Micro 4/3 is the Panasonic 20mm f/1.7 pancake that is 40mm FF equivalent. Extremely versatile.
I doubt an EF wide-angle pancake would be able to come anywhere near the optical performance of either (1) full-size wide-angle lenses; or (2) the 40mm pancake. Not enough room for corrective elements. For longer focal length lenses there's probably not enough room to space the elements without some optical magic like diffractive elements (read: crazily expensive, priced out of reach of the target market).
If any new pancake were to come out I'd expect it to be EF-S, 25mm and probably f/2.8 (though f/2 would make it far more attractive on APS-C). That would put it in line in terms of angle of view with the 40mm f/2.8 on FF, and not coincidentally also with the 20mm f/1.7 Panasonic.
You're right, short focal length lenses need to be retro-focus and that makes it a real challenge to get a good optical formula into a pancake.
As for pancakes, I had a 40mm Voightlander but to be honest I think a pancake is too small on a 5DMk* body. I prefer my little 50 and 35 mm Canon lenses for compactness and light weight while they still give something to hold on to.
5D3, 5D2 | SY14mm f/2.8, Ʃ20mm f/1.8, 24mm f/2.8, 35mm f/2, Ʃ35mm f/1.4A, 50mm f/1.8 I, Ʃ50mm f/1.4 EX, 100mm f/2.8L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12mm f/2, Ʃ19mm & 30mm f/2.8 EX DN, 55-210 OSS | FT-QL, AE-1P, FD(n) & FL primes