For me (a former M owner and future M owner someday) this is what I want:
1. As small of a camera body as possible with an APS-C or larger sensor. The camera should fit in the pocket of loose slacks, cargo pockets and coat pockets.
2. Prime lenses that are a tiny as possible,allowing the first need to be satisfied.
3. The ability to take larger, EOS lenses.
The current EOS M is very close. The major improvement I'd like to see is actually in the prime lenses. The 22mm lens is a great prime, but it's still too big. When the lens is on the camera it adds over an inch to its depth, making it a pain to put in any pocket other than a coat. Yes, you CAN put it in pants pockets, but it's awkward and a constant nag when walking, sitting, etc. jogging would be out of the question (and by jogging I mean if you were, say, in an amusement park and needed to jog after a child or were in an airport trying to get to your gate, etc.).
So, for me, I'd like to see them make the M body just a little shallower and the lens thinner, if these two are possible, so that I can actually out it in my pocket (loose pocket--shorts,cargo pants, slacks, etc.).
Everything else about the M is just fine (for me). I was satisfied with the autofocus (it's a point and shoot, not a DSLR), and the ability to take any EOS lens is just amazing. Plus, the IQ is on par with a 7D. That's fantastic.
BUT......And his is big to me: if I can't put something comfortably in my pocket and forget about it, then I have to use a separate bag. And if I have to use a separate bag, then screw it: I'll just carry a DSLR with a pancake lens.
I can operate my 6D with the 40mm pancake (pretty much the same field of view as the M with 22) using one hand. Yes, it takes me two hands to adjust settings, but for shooting I can do it comfortably one-handed. In fact, I'm able to shoot my 6D easier with one hand than my M.
Something to think about.
But, that's just me and my needs/wants.
In summary, I want a camera in my pocket. If it doesn't fit then I'm bringing a DSLR.