I want to upgrade from my D610, D750 is not a worthwhile upgrade for me and the D810 is a bit dated for me to consider. But pre-order? Seems pointless really unless you get a discount.
Heard rumors? The product hasn't even been announced, any rumors are not worth paying attention to at this stage.
Saying that. Nikon use sensors manufactured by Sony and since Sony cameras have not had any major improvement in DR there's no reason to expect any big improvement here, probably a small improvement in sensor performance and resolution in line with other offerings on the market. Considering the D810 has a top of the range sensor this isn't really something that Nikon need to be focusing on improving anyway all things considered.
My god though the screen does need an overhaul and considering the D750 I'm sure we can fully expect that to be revamped. A larger, brighter, higher resolution, articulating touch screen really is needed in 2017, and a UI change to fully utilise that touch screen functionality of course. Besides that we have the AF system, which while adequate, bundles the AF points in the centre of the screen, so I would not be surprised to see this improved upon. And then there is the video functionality, perhaps Nikon's single worst area of performance, dire, utterly useless video AF, needs a totally revamp and considering that Nikon don't have any reason to utterly cripple their product like Canon have done with the 6DII, 4k video recording.
I'd also be expecting wireless connectivity just to bring it in line with other modern offerings from the competition and Nikon's consumer range of cameras.