Update:
oops, I was too slow in my typing - or my posting is just too long 
Quite interesting nobody will take the D5X bait I put up,,, Oh well - keep on dreaming 
I assume that even Nikon was surprised by the success and market adoption of the D800 series, which basically killed the D1H/D1X, D2H/D2X, D2Hs/D2Xs and D3/D3X heritage for the D4 generation. From a business platform perspective the lower price of the D800 increased the number of cameras in the market substantially vs the D1/D2/D3 generations. With more camera in the markets and its relatively high resolution requirements triggered quite a few Nikkor sales (more than a few high priced D4X's could have done for Nikon's overall bottom line).
My 2 cents on "dreams ..."
1) Now that the D8xx series is so well established, Nikon will continue to explore and exploit this market segment. While the D800 was probably a risky step to change the market dynamics, there is no need now for continuing such an approach. My guess is that the next generation D8xx will be rather be a "conservative" step, i.e. 45 or 54MP if we talk resolution. With about the same price range (and the "mandatory" 10% uplift for a new generation).
2) This would allow Nikon still to choose the route to position a "revolutionary" D5X with an appropriate price premium against the "standard" D5. Not sure how many customers would embark on a route of paying for instance 7-10k $ for a 88MP or 100MP FX D5X Camera (or whatever resolution), without impacting the "lower cost" 3-4k D8xx successor market. Using CX cameras like the J5 with current modern FX lenses, the aquivalent resolution over the FX sensor space would be either 80 or 150 MP (depending if someone uses the J5 or V2 as base). Meaning, existing lenses wouldn't be completely out of scope.
Talking about guesses. My current guesses are rather around the renewal of the "holy trinity" lens set (AFS 12-24mm, 24-70mm VR and AFS 70-200mm).
1) The AFS 24-70mm/2.8G VR seems to be the early bird in this generational change. Electronic aperture control, VR, etc. But the 82mm filter size seemed odd to me at the beginning. As it is a very unusual filter size.
So the "conspiracy guess" is that the 24-70mm introduces the 82mm filter size as the new standard size for the yet to be completed new triple lens set.
2) There might be a new AFS 17-35mm/2.8 with this filter size, having more focal range than the 14-24mm and still allow for filters to be attached. The AFS 17-35mm was introduced with the D3 in 2007 and the AFS 16-35mm/4 never emotionally catched on the other f2.8 lenses in this line. So this success is kind of long overdue.
3) The AFS 70-200mm VR II is the lens which from an optical performance has the lowest upgrade need. Might only be a minor update with an "E" aperture control and all the new FL, nano, etc. gadgets applied. And of course 82mm filter size
rgds, Andy
