EF-s prime only doesn't make too much sense unless it's the widest of angles, but really if that's the need just get a FF setup. A super-tele compact walk-around or telephoto dedicated super-zoom with L quality build and optics I've thought of before, in a package much smaller and lighter than anything currently in the EF-L series line-up (except for the 70-300 DO which I really have my eye on atm), would be desirable for me.
The way I shoot, if I'm going to want something like a 70-200 IS, I'm probably going to want either or both, more than 200 (aka 300-400) range and or a second body with wide angle/standard angle attached which starts to bring the requirements for smaller weight and size into serious consideration. So either a one lens does it all but smaller and lighter and designed for EF-S L quality version of the 28-300, or just equal quality but speed of 70-200 IS, both of which I guess they sort of have already, but not quite. They are definitely more consumer built than I would like.
It's a hard range to pick out and not a focal range I'm concerned with shooting most of the time, not buying one until I am spot on positive in what I need, or perhaps just get something used/cheap enough to own, and then rent specifically what's best when important events require it. Again I want the 70-200 IS II speed and IQ but I also want ability to get to 300mm and greater (so tele-extender..) but then I don't want the enormous bulk and weight it comes with. I also don't love the idea of running around with 2 cameras or switching lenses, so a walk-around super zoom becomes sensible for days when shooting at any focal length is a possibility, and still again, to do that currently in the highest IQ, requires that enormous 28-300, expensive push-pull back breaking monstrosity.