I guess there is only one thing I really want changed compared to the existing 35/2 - the AF motor! It is the loudest lens I have by far.
My worry about any new version is I like the relatively good close focus distance of the 35/2, so I hope they don't make it longer. Assuming that, especially with IS too, I don't mind about double the price of the 35/2. Of course I wouldn't complain if it turned out cheaper!
Speculation: I think the existing 35/2 is too close to remain in the lineup if the IS model appears. I think it makes more sense for them to later do a budget EF-S 35/1.8 like Sony/Nikon already do.
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, 7D2, EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 10-18, 15-85, MP-E 65, Zeiss 50/2 macro, Sigma 50A, 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye