I've been a cheerleader for the original 1Dx since mine arrived just before the London Olympics.  It built on the framework of the 1D Mark IV and the 5D Mark II to make a superbly capable professional camera.  I loved it then, and it is my go to body to this day.  When word started to leak that there was a new version imminent, lots of folks asked me what I thought.  I was very clear then.  The only thing that would make a Mark II extraordinarily compelling would be a significant increase in dynamic range.  The Mark II was officially announced on Feb 1, 2016.  Let's take a look at the announcement and see what Canon has done.