The idea of a Canon EF 24-85 f/3.5-5.6 IS lens is nothing very fascinating. (Unlike Canon’s EF 200-400mm 1.4x f/4 L USM IS which I believe is a phenomenal lens!)
However I do see it might have merit if Canon can produce it at a price at least a couple hundred dollars lower than the 24-70mm L IS (which I believe is its current closest ‘competition’).
Canon already have produced a 24-85mm f/3.5-5.6 USM but without IS. See some reviews / user reports here:
It does ok, but doesn’t performing that well wide open and while it has USM, it doesn’t have IS. So, to me, it’s a new 24-85mm might be close to the same size (or even slightly smaller) than the current 24-70mm L IS, it could win if it’s a ‘budget’ option. I agree with some other posts, that the 24-105mm is still a compelling budget option too (it’s L and weather sealed, but not optically the most brilliant across the board, but it’s got decent bang for the buck).
I used to have the 28-135mm USM IS (and a decent copy of that), however I sold it after I bought an EF-S 15-85mm (much more practical zoom range for an APS-C).
I could see some people pairing a decent (new) EF 24-85mm with a 6D, if it was priced right –and had at least decent (if not stellar) optics, especially ‘wide open’.
Let’s see what eventuates!