Here is one possible scenario for the Nikon D810 replacement: a new D820 with a 46MP sensor. This is the only interesting rumor I heard recently worth sharing online. I am still not 100% confident with that information since it is coming from a single source. Hopefully this blog post will trigger some more tips and additional info on what's coming next from Nikon.
Nikon is getting ready for the CP+ show in Japan at the end of the month - see the new Nikon booth in the picture above. Any potential announcement should happen before the start of the show (February 23rd, 2017).