This post is base on deductions gleaned from the way companies work in predictable patterns. Sony has been making sensors for Nikon cameras for some time now. A year or two later the same sensor makes its appearance in a new Sony camera model. Such appears to be the case with the 36mp sensor in the D800 now appearing in the Sony A7R. This is not unusual behavior. Companies typically purchase not only technology, but the right to exclusive use of that technology for a period of time. The question this begs is what happens after the time period runs out.
With the Olympics coming up Nikon is due for a major announcement in pro cameras. The company is going to have to trump the Sony A7R with something bigger. Speculation has centered around a higher megapixel D4x. This vision derives directly from the former D3x as a high megapixel pro version camera. There is, however, one thing wrong with this vision: A high megapixel camera -- especially one higher than the D800 -- does not really gain anything by being put into a bulky D4 body. Such a camera would be unnecessarily cumbersome. My guess is that the next iteration of super-high megapixel sensor will take the form of a Nikon D800x where it makes that same sense as Sony's offering of an A7 and A7R.
My guess is that this new D800x will come out with something like a 54mp sensor and be announced early next year in keeping with the hoopla surrounding the 2014 Winter Olympics coupled with the fact that the Sony A7R will be arriving in photographer's hands in late December of this year. Nikon is going to need something big in order to jump to a new dominant level.
And what about a D4x? Well, there is nothing to stop Nikon from borrowing a trick from the Sony A7-A7R series and coming out with a 24mp D4x. That, plus a 54mp D800x, would cover all the bases and trump pretty much everyone.