A new Canon EF 50mm f/1.8 STM is expected sometime in April, although the exact date isn’t known. I suspect it could show up after the new Rebels begin shipping.
We have received two mentions over the last few weeks about a replacement for the EF 50mm f/1.2L. We’re told the latest design of the 50L is slightly smaller, and a bit heavier. The front element is bigger than the current version, and that the focus shift issue will be gone thanks to a floating element.
We think the next L prime from Canon will be a replacement of the EF 35mm f/1.4L, and that a 50L replacement will be in the distant future, likely 2016 at the earliest.