What I don't understand is when everyone here claims the full frame can't be done, why not use the same basic body of the 6D without the mirror/viewfinder? No it would not be pocket-able - my sony nex 5n is not pocket-able, but it would reduce weight and size while still functioning just like mk111.
I wouldn't go to a job without two bodies in my camera bag I just would prefer if that second or third backup camera while having the exact same sensor characteristics of the main body could also be designed in it's own unique way. In this case as small as possible for those times when I want to go hiking, street scenes, video work, mounting in unusual places and don't want a full size dslr. (funny I use to strictly use a 4x5)
After using the sony nex I've come to one conclusion there are two size cameras one is point n shoot pocket-able and the second needs to go in a bag.
Here's my analogy (I'm not a golfer or a gopher) it seems Canon keeps making all these different cameras to be all to every use instead of making each camera body unique to help with specific problems. A golfer doesn't carry around a bag full of 9 irons or (do I dare:-) a bag full of woods. Nor is their putter manufactured significantly inferior to his driving wood because it's a lesser tool.
Yes mirrorless cameras are here to stay however I disagree they need to be a whole new system, but I'm only a photographer(tool user) not an engineer.