Megapixels alone won't drive me to a camera like the 5DMKIII. Enhanced ISO performance would be great (of course additional MP might improve ISO with downsampling). What I think Canon needs to do is spend a bit more time on the user interface by adding enhanced software capabilities. There is no reason a camera of this shouldn't have built-in intervalometer, HDR, focus bracketing, configurable auto ISO, etc....
We are getting towards the point where the sensor pitch makes us diffraction limited. I know my crop sensor gets there in the mid f-stops...what is it for a 7d, about f/8 or f/11, I think? I haven't been following this topic for full frame sensors. What is the diffraction limit on a full frame sensor with 22MP, 35MP, 45MP?
However it is done, better low-light image quality is probably my highest priority. This would do the most to improve my work.