Interesting discussion...My thoughts:
I think the D7xxx series sticks around as the high end consumer product with the D500 being the "pro" version in DX...it wouldn't surprise me to see a D5S and a D500S show up whenever they decide to update the D5...
I can agree that the D750 is pretty much the continuation of the D6xx series. Maybe they will do an upgrade of of both but keep the features lower on the D620 than the D760 (?). But, do they need a low priced FX?
I like some of the ideas about the next (if any) DF version, but I don't really have any interest in that line right now, so haven't thought about it much.
I would definitely like to see an upgrade in the D8xx line and like others, I expect it will be more MP and more features...I don't really care about the built in flash as I don't have them now on my D4 bodies, but I did occasionally use them for a bit of fill now and again on my former bodies.
If it were my choice, I'd basically have 2 types of bodies...The D3xxx through D7xxx, D6xx and D750 which would all be the same physical button placement, similar physically, etc. with some differences of course based on level for the entry level to high end consumer in both DX and FX. For the D5 (and beyond), D5xx, and D8xx, I would have the exact same body as the D5 and make those with true pro level features - the D5xx being there for pros who needed DX for various projects, the D8xx for those who needed the large MP / more artistic but with a reasonable FPS / buffer. The D5 and beyond would remain a perfect photojournalist camera with high FPS, buffer and low light.
I know that you can shoot sports or artistic shots with any of the cameras in the lineup from a D3xxx up, but separating out the pro bodies will allow them to make each the best they can be for their intended pro audience.
Oh - and BTW, my D4 is finally on its way back home from repair! They had to wait for some parts. It is supposed to arrive tomorrow! Just in time for my first Friday football game! And for Opening Day on Saturday for the local rugby team! Looking forward to having both bodies back! On the other hand, with a wedding coming up in a few weeks, I was thinking about telling my wife I needed to buy a D5 and then "settling" for a D500 ! I probably would have ended up renting a body though...