I agree with most that the new 50mm IS will be f/1.8 based on history, and of the available Canon patent which is a 50mm f/1.8 IS. There is no Canon patent of a 50mm f/1.4 with IS.
In terms of which lens it will replace, there is a good argument for both.
First, lets look at why many buy the 50mm f/1.4 over the f/1.8:
* Superior build quality - 50mm f/1.8 II is super plastic, 50mm f/1.4 is more solid (but does have its own issues)
* Superior autofocus - f/1.4 has superior USM over the f/1.8 II's micro motor.
* Better image quality - f/1.4 is sharper with less falloff
* Faster aperture - f/1.4 is faster than f/1.8
HYPOTHETICAL:
Now, lets look at a potential 50mm f/1.8 IS if it follow the trends of the 35mm f/2 IS.
-The 50mm f/1.8 IS will certainly have better build quality than the 50mm f/1.4, which has a history of autofocus reliability issues and simply isn't built to the same standard as the new IS lenses.
-The 50mm f/1.8 IS will likely have faster and quieter autofocus than the 50mm f/1.4 if the 35 IS is any indication.
-The 50mm f/1.8 IS will likely have slightly better image quality than the 50mm f/1.4, with modern touches such as curved aperture blades and newer lens coatings
-Obviously the 50mm f/1.4 can do f/1.4, while the 50mm f/1.8 IS cannot
-However, the 50mm f/1.8 IS can do image stabilized video, while the 50mm f/1.4 cannot
-The 50mm f/1.8 IS will likely stabilize at $549, versus $349 street for the 50mm f/1.4
So, the 50mm f/1.4 I believe will become a much less attractive lens, with price being its main draw. But, those truly fixated on price may be attracted to the cheaper 50mm f/1.8 II or 40mm f/2.8 STM instead. And, those wanting the better build and more modern features & design will be attracted to the 50mm f/1.8 IS. Those wanting the ultimate in aperture will be drawn to the 50mm f/1.2L. Those who do video will easily elect the 50mm f/1.8 IS. Thus, I believe that the 50mm f/1.4 will be stuck as an unpopular middle option if all four lenses are to be sold - I am not sure there is enough market to support four different 50mm prime lenses.
So, I see three possible results of this.
1. The 50mm f/1.8 II is discontinued, and the price of the 50mm f/1.4 is further lowered to $299 street to bridge the price gap somewhat. The 40mm STM f/2.8 is positioned at $149 street to entice what would be former 50mm f/1.8 II buyers.
2. The 50mm f/1.4 is discontinued, and the 50mm f/1.8 II remains as a cheap "gateway lens" to get people into the habit of buying lenses; remember though, the 40mm f/2.8 STM can already serve this same purpose, so this is not a necessity.
3. The 50mm f/1.8 IS is added to the lineup and nothing is discontinued. But wow, that is a lot of similar lenses.
Thus in table form, I could see:
OPTION 1 (more likely):
50mm f/1.2L - $1599
50mm f/1.8 IS - $549
50mm f/1.4 - $299
40mm f/2.8 STM - $149
OPTION 2 (less likely - too much overlap at $100-150, too much gap between $150 and $500)
50mm f/1.2L - $1599
50mm f/1.8 IS - $549
40mm f/2.8 STM - $149
50mm f/1.8 II - $109
OPTION 3 (less likely - too many similar lenses)
50mm f/1.2L - $1599
50mm f/1.8 IS - $549
50mm f/1.4 - $349
40mm f/2.8 STM - $149
50mm f/1.8 II - $109
Of course, all speculation, but that is the fun part!
