I can tell you that there's probably NOTHING common between a D200 and a D800.
I have thought, more than once, what Nikon should do is make a "perfect" camera body, with all lessons learned in the making of a good SLR body implemented in it, with all the electronics meant to be replaced as a unit with the latest and greatest technology being installed and the old version removed when it becomes obsolete. Most of the manufacturing cost of a professional grade DSLR is in the body, with the sensor assembly coming in second. The rest is relatively inexpensive.