9VIII said:
Oops, I voted assuming a 50L, non-IS.
50mm with Image Stabilization? Well, they have a 35f2IS and 24f2.8IS, I suppose a 50mm IS isn't totally insane, but there was already a 35f2 and 24f2.8 before the IS versions came out. If anything I expect a new fantastic plastic, nifty fifty, 50mm f2IS STM at $200, but that's the only scenario where I can see IS being implemented on a 50mm lens.
Count me highly skeptical about a regular 50mm IS.
Other way around for me. I see all the non-L midgrade USM lenses getting IS at a slower aperture than the L, but the L's won't get IS.
So it will be:
50 f/1.8 STM - no focus scale, MF by wire, slow focus speed, telescopes in an out to focus, etc.
50 f/nooneknows IS USM - general purpose workhorse; exactly like the 24/28/35 IS lenses as far as design/features go.
50 f/1.2L (or f/1.2L II) - a lens built for f/1.2 - f/2 use, weather sealing a red ring, etc.
(and yes, I left out the 50 macro, I always do

)
Everyone's just so touchy about what f/nooneknows will be. If it's f/2, people foolishly say "why bother?" and forget the massive, massive upside of USM over STM for stills. But if it's f/1.4 -- i.e. a straight swap for the current old 50 f/1.4 -- people get bent out of shape that it will threaten 50L sales. Whatever.
I don't really care what it is, f/1.4 or f/2 they get my money. Reliable/fast/modern/consistent USM AF + small + sharp + IS = a winner for my needs.
- A