I would really like this pancake lens to be f2.0 or even better f1.4 ... also if it would make it a little bigger ... that would make an awesome lens for natural light under difficult conditions and still light to always have with you on a FF body ... f2.8 is too slow here ...So, you basically want the 35 f/2 or the 35 f/1.4L....Canon already makes those lenses for you. Unless the f/2 is no good and you absolutely have to save that extra 3/4" in lens length; or 7oz is too heavy.
I'd imagine one of the few ways they could make a pancake f/1.4 lens would not involve an EF mount, which would piss people off more than this thing being f/2.8. Or it'd cost more than the 35L, cause it'd have to use some specialty glass to accomplish as much with fewer elements
Also, its not like there are a wealth of low-light pancake lenses out there. I think Samsung has a 30mm f/2, Panasonic has their 20mm f/1.7, there's the Voigtlander 40mm f/2, what else? Most I've seen are at f/2.8, probably for a reason