My 2 cents from a Pro advertising, fashion & corporate perspective is.
Canon we really need this, but we need it to be really well designed and produced, with pixel performance to match.
That means 40MP+ and most importantly at least equal to or better than the D800 sensor in DR and shadow noise banding (when pulled up) Anyone who's used a D800 will tell you its fantastic you can lift the shadows 3 stops and no banding or breaking up or colour noise. Even many Hasselblads and Phase Ones are not as good.
Lower noise at ISO 50-200, would also welcome lower noise around ISO 200-800. And tbh I wouldn't expect a really clean file above that.. I myself don't shoot actual work that needs to be super high res in dark environments anyway.. Often I'm being paid because it's all about lighting and how I can make a scene look and feel.. not just capture it.
Flash sync to be 1/500 sec min, most guys like me use proper studio / location flash a LOT and being able to control the mix with ambient light would make it a MF killer.
Thunderbolt or USB 3 is a must and some really good built in wifi could be welcomed that could facilitate realtime raw file transfer ( but we are maybe still a couple of years away form this)
5 FPS is prob enough for a camera like this, with current 5D III autofocus also.
Form factor I don't mind but at least a 5D III size and feel, I think I'd prefer a 1D but slightly smaller vert grip and lighter. But I would like it feel very solid, tactile.. quality rubber and plastics even nicer buttons esp the main top shutter speed wheel. And sound like a proper camera.. cluunk! not a snap klackety klack (coughs 5D III boo)
Video.. No I've got my 5DIII for that which I dont use that often for video anyway. Unless their R&D could get a 2.5K or 4K clean uncompressed out like Magic lantern have, but I suspect this would completely cut into 1D Cinema sales.
Cost.. we dont mind say £4-5K for a good piece of kit and would rather that than a D800 direct competitor/ price buster thats cut down on features and quality.
I strongly believe if they met these specs then EVERY PRO ( in Advertising, fashion and corporate) would instantly have one in their bag. In the UK most other pros in this sector use Canon, often alongside Hasselblad or Phase one. But everyone prefers to shoot with their Canon, and importantly everyone loves the colours from RAW file more than Nikon and it's also the quality of L primes that keeps us on Canon. And very importantly its the user interface is much nicer than Nikons, simply quicker plus the C1-3 are invaluable too.
Now to all the haters here, if you don't need the MP or maybe would like one at 5D price then you're not getting it.. what Canon are hopefully taking their time making.
Nikon went 'hey look a 36 MP sensor thats great'. Yes it really is a great sensor but the camera is much much a lesser tool, the 5DIII is very very good in that respect and thats what Canon have been working on, and probably trying to hang on to their current sensor process as long as possible.
And Canon know there's a big proper pro segment of the market that would love such a camera, but I'm slightly nervous that Canons Marketing will want to jump on the D800/ amature photo pixel peeper bandwagon that actually don't need such a camera..
Yes what many say is 22MP is FINE for most situations, even many pro ones. BUT when we do a job that needs to be used billboard size or for a wall sized shop display then 22MP starts to break up, also under heavy stylish processing and retouching which is very often required in our work we really start to notice how the file starts to get noisy and break up.
If there are any Canon staff reading this.. please GET IT RIGHT for the PROS with this one - read my specs wish list and what everyone else is saying most importantly NOT JUST MORE PIXELS.. BUT BETTER PIXELS, then release a cut down smaller cheaper version in a year or so to keep all the amature pixel counters happy (90% will never actually need it tho, just think they need it!)