I can't think of a major change needed in the D500 because it is class leading by a wide margin and has no real competition. The lack of popup flash was a great move and will be, or should be, the case with every camera considered pro. The lack of popup flash allowed the larger VF that is the best in the crop camera industry. No one with need of a pro level camera would use pop up flash as a light source but would use it as a trigger for the optical CLS. But CLS is pretty obsolete, and limited for any but very casual use when low cost reliable RF controllers are available. Nikon should replace optical CLS with a new standard RF protocol. Adding a 2.4ghz RF module is very cheap now, about $1 in off the shelf quantity buys.
A D510 could be expected as a refresh, with better Snap Bridge, maybe 1-2 more fps, but there is no real demand for missing features. Maybe more use of camera control from the touch screen but that is firmware and not needing hardware changes. They have much more important uses of R&D funds, like the mirrorless.
After shooting with a friend's Sony high end model I am not impressed with the lack of accessible controls and very bad ergonomics.I could not imaging trying to make changes fast enough in event shooting or BIF to make it worth the bother. IF and WHEN Nikon adds mirrorless, I sure hope they retain the feel in hand and access to settings without menu diving like is required with Sony. Canon is not much better than Sony in the number of menu levels required for some basic often used settings. Almost all the mirrorless options out there ignore ergonomics and concentrate on small size assuming the market wants a pocket pro camera instead of a usable pro camera that just happens to be mirror-less. If Nikon makes it any smaller than a D500 or D850 they might turn off the pro shooters. Reducing weight surely would be welcome but it has to fit a normal hand, be suitable for grip addition, and have a large number of direct controls that reduce menu diving during any session.
What is needed for lighter mirrorless cameras are lighter lenses. By using modern optical theory, and composite material lighter, smaller glass is possible. For example a 70-200 with 23 heavy elements is a good candidate for more modern lighter materials and fewer elements. They would sell a lot of lenses if lighter versions were introduced when they introduce the lighter mirrorless. My smallest bag, with 1 D800, a 70-200, 24-70, and 14-24 or wide prime, plus a SB900 or two is over the weight limit for carry on luggage. Lugging it around town walking for miles is a pain but for events, what else get the results?
I really do not need a new camera but they added enough excitement to the D850 without raising the price that I see I will probably get one, and mirrorless will probably be announced the next day. About the only feature of mirrorless I would really benefit from is silent shooting. The pro Nikon's are loud and restricts their use in theater venues. My D7000 is so much quieter that the administration of the main ballet theater here, the famous Mariinsky Theater, gave me a certificate saying it was quiet enough to be used from specified positions while my D800 could not without mounting in a sound absorbing container.