I can see both the 50/1.2L and 50/1.4 being replaced. The 50/1.2 because of it's general performance, and the 50/1.4 because of the competition (Sigma).
Also a 50mm-prime could be very nice for the budget handheld video-shooter who is into the short DoF-look and all that, but they really should keep the 50/1.8, because value for money and all that.
I wish Canon to replace the 50/1.4 by a 50/1.4 IS, because it would make life easier for me choosing lenses
(Because I don't want to think about Sigma 50/1.4 vs old Canon 50/1.4 vc Canon 50/1.8 IS.. assuming the price range remains on an equal level