It's difficult to make a 50mm 1.4 with IS, because the IS-Group is of course enlarging the whole formula. So, if the lense gets longer by 10mm, the focal length mostly get's longer, too. And with the same optical calculation (and therefor aperture-diameter) the f-number increases.
I would really like to see a new 50mm 1.0, just to show the finger to the competitors (for fun
, but after all I even stop down my 85 1.2 II. Scenes where you really need f1.2 are rare. I mostly use the lense @f2 or f2.2... so a 50mm 1.8 is quite nice for portraits.
If you need the ultimate shallow DOF, use a larger sensor