We might see a 24-70 f/2.8 IS in the next few years, but as others have said, I would expect it to come with a pretty significant price, say a $400+ premium on whatever the current price is of the 24-70 II.
As the camera market shrinks, it will impact resources. I would expect Canon to focus even more closely on lenses that they believe they will sell at both high margin and volume, i.e. give the best return on investment.
Honestly, I am interested to see what Canon does next. If you look back, their strategy in the "L" range is pretty clear: industry leading zoom lenses. They've given us 2 new UWA zooms, a UWA fisheye, 2 new general purpose zooms, 3 relatively new short range telephoto zooms (70-200 ii, 70-200 f/4 IS, and 70-300 L), a longer range tele zoom (100-400 II), and replacement big white lenses (including the 200-400), with one exception, all since 2009. All of those lenses are the best of class.
Really, I could maybe see a UWA f/2.8 update, a 24-105 update, the rumored superzoom.....updates to a few primes (14, maybe 24, 35, 50, maybe 85, maybe the 135, maybe 180 macro).....and, of course, I'd love to see something creative (f/2 zoom?, 400-800?)....so there is still room for releases....but if you think about it in terms of return on investment in a shrinking market....I can also see Canon really slowing down. Maybe 2-3 updates a year as they have a great industry leading lineup, especially in the zooms, in the high volume areas.
That is a long way of saying, I really do not see a 24-70III on the horizon.