i just hope they keep the integrated lens hood, I love the 300 F4L IS
More modern IS and full weather sealing would make great improvements as well as the more modern coatings supressing flare and CA, depending on cost I would look at an upgrade. although the sigma 120-300 f2.8 is really tempting even if i got it the 300f4L would stay just for its super well balanced and convenient sized when used with a 5D body