Personally, for both event/PJ shooting stills and for my upcoming (hopefully) Ursa Mini 4.6K EF (1.45x CF sensor) I'd kill for a 15-60/70 f/2.8. The wider range isn't isn't quite a as critical for me on the 4.6K as on the C-series since with the wider field of view and much higher resolution, I have basically a 2x built in TC and a 0.85x wide angle adapter when delivering in 1080p if the lens is sharp, and still at least some wiggle room in 4K/UHD, but its still pretty handy for doc and run and gun like I'll be doing. For event and PJ work its just killer, and in many cases might let me haul out only one 7D instead of two.
The biggest thing I'd want outside of more range is easily mechanical/build quality; something on par with the 18-35 would be ideal and would put it head and shoulders above all the other crop midrange fast zooms which match up very closely with the current 17-55 on pretty much everything. Specifically, a large, longer throw, damped, front mounted focus ring, solid finish (despite its more than adequate optical quality, both the 17-55 and sigma 17-50 don't exactly send a strong professional vibe to clients, nor to me with regards to their durability), and being roughly parfocal and with lower focus breathing is what I'm after.
After that would be a stop or two better IS and a little sharper wide open with better coatings for better transmissions, but those are all minor compared to the mechanical quality and range. Right now for me its down to keeping my current 17-50 or switching out for the 17-55, bascially all on the merits of focusing--basically whether to plunk down ~$200 net to trade up to the 17-55's with almost double the throw and smoother operation (plus a little better build quality and range and more reliable focus for stills), relative to my current 17-50 which though has hard markings and stops, is a tad wider and properly front mounted. But both pale in comparison to that of the 18-35, so I'd rejoice if Canon gave us something on a similar level just for that.
The physical limitations would preclude a reasonably sized and priced 17-55 (or more) f/2+ with IS, as much as I'd like it, so I'd be content with the greater range to give me more of a reason to have both the f/2.8 and the sigma in my arsenal, or maybe even just the former for now. A good while back Tamron came out with a 28-105 f/2.8 which is the rough equivalent of a 17-70 on crop, so if Canon could make something like that a bit wider (and sharper obviously) with IS and 18-35-level mechanics, I'd buy it in a heartbeat even if it was fully $1000.
