I have been holding off this post for few weeks now, hoping to get some additional information. Well, I did not and since it has been very slow on rumors, I decided to post this anyway. As of today, take it with a grain of salt:
I received a tip that Nikon is working on a new D810s camera that should be announced soon (in the next few months). I do not have any details or specifications, but I am guessing that compared to the D810, the new D810s will have a better high ISO performance and maybe few other improvements. It seems that Nikon is not yet ready for a big jump to a D850/D900 camera which explains the different D8xx variations (D800/D800E/D810/D810A/D800S). They also know how to "squeeze" better ISO performance out of existing sensors, so a D810s makes sense given the current "wait and see" approach.
This information comes from a reliable source who previously told me about the D810 camera. Like I already mentioned I have not received any other D810s tips, which makes this rumor questionable.
Several companies are rumored to announce new products at or right before the PhotoPlus Expo in NYC (October 21-24). This is also the last chance for Nikon to introduce something new before the holiday shopping season. All this is just speculations - as of today I have not seen any indication of an upcoming product announcement.
Source: http://nikonrumors.com/