Another vote for an ultra wide zoom that is sharp at the corners. Don't get me wrong I like my 16-35 but it leaves a lot to be desired in the corners.
Given the minimal improvement between the 16-35 I and 16-35 II despite many years passing and increasing of front element to 82mm filter thread, I think the only way a UWA Zoom will be sharp at corners is with a bulbous element w/o filter support.
Again, bulbous elements have very good filter support..............
All in vain.