There are a considerable number of 5D mkII shooters already using Magic Lantern, Canon also has to consider that. That whatever they improve on the Mark III has to be better than 5D mkII + Magic Lantern Unified.
5D mkII+ML Unified already has these and more;
-Headphone monitoring
-Focus peaking and other focus assist features like;
-Magnified window
-False color
-Histogram while recording
-Waveform while recording
-Clear screen HDMI out for use with ext recording device (Im careful not to use the term Good HDMI out signal because I'm not sure how good it is)
-Split audio channels with independent controls allowing use of internal and external mic simultaneously
-HDR video (albeit tricky to use with fast motion, ok with 60D, 600D due to their 720 60P)
-customized crop marks
-Built in Intervalometer for video
-Variable frame rate from 3 to 31 fps which is good for clean lowlight/night shots or for motion blur effects
-Programmable rack focus
Also, for much less cost than a new DSLR, the mosaic filter has solved Moire and Aliasing on the 5D mkII
There are also several features for stills like focus stacking and bulb ramping.
So with these features in our current 5Ds I for one am looking for significantly more improvements in the new 5D mkIII than just merely improving on a stock mkII.