I am not certain there will be another increment in the 5D line.
If you remember film, there was the 1 series of pro cameras, the Elan/30 series of advanced amateur cameras and the Rebel/100 series of economical cameras. Then they brought out the EOS 3 which was between the pro and advanced amateur lines. It was such a good camera that many pros started using the EOS 3 instead of paying more money for a 1 series. Canon never did an upgrade to the EOS 3, although they upgraded all their other lines.
Is if possible that Canon will find that the 5D has interfered with sales of the 1Ds line? If so, will they do the same thing that they did with the EOS 3 film camera and leave it in the line, but not bring out any upgrades?
This is not a prediction. Just me looking at my EOS 3 and wondering why there was never an EOS 3N.