As long as we are speculating, I expect the 50 1.2L USM to be replaced by a 50 1.2L USM Mark II. There are too many wedding photographers who love this lens to let it fade away.
A 58 1.4 IS will either replace the 50 1.4, or never materialize. I'm still expecting Canon to compliment the 28 2.8 IS and the 35 2.0 IS with 50 IS (or 58 IS) and 85-100 IS lenses. The 50 1.4 is too popular to let it fade away. But, it seems a bit crowded to have a 50 1.8, 50 1.4, and a 50 1.2. Also, I'm not sure that there is a market to support both a 50 1.4 and a 58 1.4.
Personally, I'm waiting to see what Canon does to upgrade the 50 1.2L and the 50 1.4 before adding a 50 prime to my kit. My current needs don't justify a 50 1.2L, so I'm kind of hoping for a 50 1.4 IS to compliment my 35 2.0 IS. But, I'm actually more intrigued by a 58 1.4 IS as a compliment to the 35.