24-70 f4 IS. Hmmmm interesting.

vs. 2.8 24-70 Lose one stop, pick up IS, save $750
vs. 24-105 lose 35mm of zoom, lose $750 - must pick up crazy sharp????

So push ISO +1 to recover the shutter speed for the interior shots of the ever moving grandkiddos - subject movement

Pick up IS for when no one is moving too fast - camera movement...oops already have that w/ 24-105!

This will have to be crazy sharp, crazy fast AF, AND come down in price.

I am not thrilled with the softness in the corners of my 24-105 - but not willing to go minus 35mm in zoom and minus $750 for a little corner softness.

Perhaps the 1dx has the ISO performance to start at f4 vs. f2.8 to shoot +1 on the ISO for interiors.

My two work horse lenses are the 24-105 and the 70-200. I like the long overlap in focal length as it allows me to use my onestepzoom feature on the margins before having to swap lenses a great deal of the time. I pick the predominant lens and then only shift when my setting shifts i.e. in/ex-teriors.

If it is crazy sharp it might be a landscape zoom?