This is a fascinating thread. I've never seen so many people deadset on what they think this new lens is, and what it means for future offerings.
My ongoing list (from all of you) of what this lens might have been made for (besides making Canon money):
1) Video focused, or perhaps more specifically, a still lens that has been modernized to better support DSLR video (STM)
2) Value focused, an inexpensive L lens in the vein of the 17-40, 70-200 F/4, etc. There has also been chatter on this point that it might be non-L akin to Nikon's 24-85 or Canon's non-L 28-135 -- a reasonably priced standard zoom option.
3) Form factor focused -- a lighter and shorter lens than the large/heavy 2.8 standard zooms. There were a few comments re: smaller sizer allowing -- much like the 70-200 F/4 glass -- smaller / more common filter diameters than their 2.8 counterparts.
4) The new kit FF lens (either just for the 6D or all FF bodies), and in the vein of the (much cheaper) 18-55 EF-S, this becomes the standard length zoom that no one actually buys by itself. In that sense, this new lens isn't competing with the higher end 2.8 zooms -- it's just another lens with a red ring out there. That red ring, some folks astutely noted, may become the tantalizing extra that draws people into FF with Canon's 6D rather than into FF with Nikon's D600.
5) Far better IQ than the 24-105 IS. This plays part and parcel with the notion that this 'family' of new 24-70s is intended for the upcoming high MP bodies we shall see down the road.
6) This new lens will replace the 24-105, eliminating the 70-105 overlap that current lens has. This is a deliberate move by Canon to sell more 70-200 glass. (I have poked a few holes in this theory, but it's as plausible as anything else on this list).
...and it obviously could be a combination of a few of the above points. Time will tell.
And for those calling this a rumor, this is a CR3, and I have faith in our moderators in that call. :-)
Excellent discussion, all.