I best get saving....For me, the f2.8 aperture is more important than size or Image Stabilization. The current mkII has the best sunstars of any lens I know...I just hope Canon keep that the same...although I suspect they will follow the f4's route and put more blades in the aperture than necessary.
A flat plane of focus at f2.8, low CA and newer and better coatings would really help this lens. The current mkII is already very good for flare control (way better than the 24-70IIL at 24mm), but like all things...there's room for improvement.
I was pondering this the other day. Canon used to get a bad rap for their wide lenses. But these days, they have the most interesting, innovative, creative and capable wide lenses in their lens portfolio...more so than any other brand. The fish eye zoom 8-15L....unique and the last fisheye I'll ever need. The TS-e 17L...again unique and a fantastic optic. The new 11-24mm L is the widest rectilinear zoom lens ever made and again it's an amazing optic. The 16-35 LIS f4 has won a lot of hearts for it's impressive optical capability...so an f2.8 version should be very welcome too.