Anyone else fancy this lens?!!
YES! As you can see from my focal length listing it would be the lens to fill the gap with a walk around zoom. And the combo 15-70 and 70-200 @ f/4 + with IS would be a extremely versatile one.
You say f/4 and that is IMHO the right way: It should be possible to design a zoom with f/4 and a focal range of 15-70mm with great image quality at a moderate price tag of ca. 800 $ / EUR ... at f/2.8 it would be 2000 $ / EUR for the same image quality and the lens would be to large and heavy as a walk around lens to be with you (nearly) all the time.
Best - Michael