I think 20-70 or at least 20-60mm range would be extremely useful. I would sell both my 17-40 and 24-105L straight away for such a lens and keep only 70-200 F4 IS. These two zooms (20-70 + 70-200) would be enough for my needs. When needing wider than 20mm, Irix 15mm would be my lens of choice.
Somehow I doubt it will come true though... but maybe Sigma or Tamron decide to make something like that?