I don't think Nikon will produce a digital back for the F6 or any other film body, but hopefully I'm wrong.
A digital (perhaps even b/w) FM-like body? Maybe, but I'm not convinced.
Digital rangefinder? Again, I don't belive it until I see it. The market is probably too small, but Nikon have done crazy things before (new versions of their 1950's S3 and SP rangefinders, for example). We'll see.
Nikon will continue to develop and sell small Coolpix-type cameras, but have already stated that they will focus on digital SLR's. That's where the real money is. Still, Coolpix-type cameras are good for us all, since it allows Nikon (and other companies, for that matter) to develop and test new technology in a, shall we say, less risky environment. A good example is their current P1 and P2 models, with built-in wi-fi capabilities.
I expect to see a full frame DSLR within the next two years, or so. There will also be several new VR lenses (long telephotos), fast DX lenses (e.g. 20mm f/1.8), and so on. Obviously, there will be a new D3-series at some point. Perhaps the D3X will be Nikon's first full frame DSLR? There's also a good chance that the F series will live on. I'm not surprised if the first full frame DSLR will be named Nikon F7. The heritage lives on.
Also, I expect Nikon to develop new technologies to combat current issues like sensor dust. At some point, I believe most (semi-pro/pro) models will include built-in wi-fi and perhaps even GPS. I've used the WT-2 with my D2X. Very cool, but not very elegant.
Other technologies we've already seen (in Coolpix cameras) includes face recognition. I'm not convinced this is useful for a professional photographer, but I said the same thing about autofocus lenses some 20 years ago.
Other things that springs to mind: In-camera anti-vibration technology, higher sensitivity for low-light photography, improved battery technology, even better flash systems, and so on.
Perhaps we'll get low volume special versions of popular models, including monochrome and infrared cameras. Canon already have a model customized for astrophotography (EOS 20Da). Normally, I would have expected such a camera to come from Nikon (first).
Ending production of film models and manual focus lenses was a natural thing to do, and doesn't necessarily mean that Nikon will spend a lot more money on R&D. Regardless of this latest decision, I except Nikon to continue to develop more great cameras and lenses in the future.