Canon 24-70 f4 IS ???????????

whoa... at 24mm? how about less distortion and actually sharp corners. I mean just sharp, not sharper than 24-105 because the later put the bar very low.

add better coatings to reduce flare, improved bokeh (24-105 sucks when comes to busy OOF areas), less weight and smaller dimensions, i.e. closer to 17-40 than 24-105.

oh yes, there absolutely is room for improvement with 24-105/4. not that it was a bad lens, but let's not pretend it was flawless..