It is on schedule Nikon wise for a D5 to be released in the first quarter of 2016. It has been 4 years since the D4 was released and it came 4 years after the D3 which came a little less than 3 years after the D2x, though Nikon was under the gun to come out with a full frame DSLR with Canon selling great numbers of the 5D starting in 2005. I switched over to Canon to get the better high ISO performance and better autofocus performance their cameras provided. I switched back to Nikon with the D3 to get a pro build full frame camera and the new Nikon zoom lenses developed for this camera though at the expense of mediocre autofocus performance.
The big gain with the D5 will be the greatly increased number of autofocus sensors and hopefully much greater viewfinder coverage with cross type AF sensors. This would be reason enough for me to add one to my kit. My wife has the Olympus E-M1 camera and I was surprised at how much better the autofocus system was than that of my D810. The camera's electronic viewfinder is also great as I can see in near darkness and not take a shot and then check the rear LCD to see how it turned out and with the electronic viewfinder the camera shows what different EV settings will do before the shutter is released which is way kool.
A lag-free EVF (or one with imperceptible lag) would be fantastic.
I'd say a more useful thing than simply having more AF points would be more intelligent AF - things like combination AF for eye/face tracking, using pattern recognition or manual selection for acquisition and densely-spaced PDAF points for tracking. Without some way to actually use all those AF points, selecting between 400 different points just becomes too time-consuming, and all the AF dots end up blocking up a lot of the viewfinder.
A bonus would be a shutter as quiet as that on the D810. It is also time for Nikon to have built-in Bluetooth and a built-in GPS receiver. I can turn them off as I do with a smartphone to conserve battery power but I should have the option and this functionality should be provided on a $6500 camera and not just the $300 P&S cameras like the Nikon Coolpix AW130. It would be nice to have both card slots use the same type of memory card which ended with the D3 and is not found only in the D7200 and D750 cameras.
What would be useful is a USB port for charging, instead of having to take out the battery and put it in a charger. Quite often, a single battery will last all day, and USB chargers are now ubiquitous. Not to mention the many portable lithium-ion phone/device chargers which use USB, and which themselves can be charged via a car charger or solar panel. Add to that USB or WiFi transfer for transferring select files to phones, tablets and other devices too.
For wedding, wildlife, landscape, and product photography I have never needed to use a ISO setting above 6400 and 99% of the time I am at ISO 4000 or less. Making a night scene look like high noon with ISO 120,000 results in images that kill the mood and distort the scene. I can see it for sports which is artificial to start with considering the artificial lighting and artificial grass but that is about it.
I'd probably go up to 12800. Sometimes you just need that extra bit of shutter speed to capture a moving subject, or to prevent star trails from appearing in the night sky.
That said, output needs to be as clean as possible - a clean ISO 6400 or 12800 is far more useful than the ability to shoot at ISO 204800.