Your post has given me pause to think about this DX thing some more. It must be a slow news day around here.
I've recently used my daughter's D5000 and it's kit lens and I've recently just returned from another long trip abroad where my only camera was a DX Fujifilm X100. That's a very capable but photographically restrictive camera with a single focal length 35mm (FOV equiv) lens.
Where you have enjoyed migrating to the larger FX, I still romanticize about a smaller fully capable robust travel camera. While I don't treat my cameras harshly, I do use them and especially in some of the travel I'm doing now really appreciate a camera that can take some rough handling. Not so much as to survive that rough handling, but one that I don't have to be preoccupied with how I protect it 24/7 hopping on/off public transportation, scrambling through narrow openings in caves, rickety stairways, crowded everything, etc. The X100 is remarkably robust and easy to manage in those situations. My D700 is robust but certainly in the way and difficult to secure in a nose-to-armpit tram somewhere in the world , plus after a 10 hour travel day of on your feet "get-these-facts-straight" touring, it's a load.
So after all that, my new DX camera would be a D5000 sized camera, built like a D700 with a 16-85mm f/4 VRII lens built with the same construction as my 16-35mm f/4 FX VRII. Plus a 28mm f/2 AFS VRII pancake lens (yes I want a quality 40mm equivalent general purpose lens). I don't need the "reach" in a travel camera, but I do need some WA capability and a way to reduce the size of the camera/lens for some outings. So my desire for DX in this application is to allow reduced size in the camera body and lens. Sacrificing some of the small size for robustness in design. A 16MP sensor would be completely satisfactory for my needs, but I know marketing would have likely make that something that would be marginally useful for me. Perhaps if the ISO performance criteria were stringent enough the sensor would not grow to some abdominal number of pixels.
For features, the highISO performance must be about that of the D3s, the AF performance of the D3s would also be nice.
Travel snaps or street pictures are fleeting opportunities and an easily handling fast AF is crucial. I guess if you could design a robust zone focusing system for the camera that could be very useful in these situations. I've found the unique manual focus option on the X100 to be surprisingly useful. Expanding on that, I could envision an AF subsystem that allowed the user to select zone focusing and point the camera at a focus target that was at an appropriate distance for the current situation/venue and press the AF button, pre-focusing the camera at that point (presumably that setting would be locked in until released by the user). Then using preset ISO limits the aperture is calculated giving a usable DOF or hyperfocal if allowed by the parameters. The ISO value could be allowed to "float" so that a hyperfocal value could under those restrictions be set. Of course there are several programming options, but it could be restricted to hyperfocal only, preset DOF, preset aperture (and take your chances and as normal). Just a thought, while I'm thinking and I'm aware this isn't a thorough description of what would be required (e.g. mode dependency).
Battery supply is also important and I'd suggest a slimline vertical grip option that would allow multiple batteries to be stacked. I find a vertical format is often desirable in the travel situations.
Nikon seems to be on a DSLR more than mirrorless path. Many of the features I've described here are available in the NEX or Fuji XE-1 mirrorless cameras. The XE1 has a very robust fit and finish (I'm not sure about it's weather proofing however). Nikon has a lot of expertise in building robust systems. And frankly their top consumer products that use composites are quite durable products. I'd like to see Nikon more innovative it taking those design and manufacturing skills and target some smaller cameras as the alternative highly functioning imaging device (e.g. non-DSLR) is moving rapidly into another space. It may be too late to expect more DSLR options as the alternatives perfect their low-light focusing and raw focusing speeds, which are two reasons for preferring the Nikon DSLR. Nikon still has a strong edge in predictive AF capabilities with a highly perfected AF system, but as peoples expectations from an AF system are replaced by the inherent DOF small sensor systems this may no longer be an advantage.
Roger It's still, ISO, aperture and shutter-speed, right?