Well they managed f2 with the 35 IS so any upcoming 50 should be at least as fast, and hopefully faster!
I doubt that they can make an f/1.4 lens with IS. I hope to be proven wrong!
It is technically feasible, a 50mm lens is one of the easiest to design, a IS for f/1.4 would be small and lightweight, just expensive. I doubt that too many want a $1500 lens.
I believe that it is technically feasible, in principle, but still challenging. IS is forced decentering, and they need to design a fast lens which preserves its good IQ even after some decentering. It is a much harder thing to do than a non IS fast prime, if possible at all.