We've riffed on this a few times before. Regardless of what you all want, here is what you are going to get in the next 50mm prime - a 100% similar version of the recent IS primes in a 50mm package:
- Sharpness should surpass the 50L (remember, the F/1.4 lens is currently sharper than the 50L already at a few apertures)
- Modern USM, Very quick AF
- Internal focusing
- Vastly improved build quality over the lens it is replacing (in this case, the F/1.4). I own the 50 F/1.4 and the new 28 F/2.8 IS and it is night and day for BQ. Someone nearly hit me when I said the new 28 was an L lens minus the gasket, but I stand by that statement. These new non-L primes are nearly as well built as my 100L macro, but not tank-like like my 70-200 F/2.8 IS II.
The above list is a hammerlock going-to-happen list. Why am I so confident? They are refreshing all non-L primes right now in this exact fashion. They've done the 24, 28, and 35, so the 50 and likely 85 are next. These are twenty year old lenses that need refreshes.
The only debatables are max aperture and filter size. My vote of what we'll get (not what I want, per se) is F/2 and 67mm. As the 24 & 28 IS refreshses got 58mm and then the 35mm F/2 IS got stepped up to 67mm, similarly stepping up the 50mm F/2 IS to a 67mm filter makes sense. Just a hunch on aperture, though.