More diffractive optics
Canon seems to still invest significant money into developing their diffractive optics technology. Currently Canon has 2 DO lenses, the very good 400 f/4 DO IS and the decent 70-300 DO IS.
Making long lenses that are smaller and lighter will always be desired, especially with the boom in digital cinema cameras and wildlife photography.
More patents have appeared, one for a 600 f/5.6 DO and another for an 800 f/5.6 DO. The 800 would measure a miniscule 46cm in length.
Chances of coming to market? I think it's a pretty solid bet we're going to see new DO lenses announced sometime. When? Your guess is as good as mine.