First, I think a modern, digital ZX-5n would be great. A modern DSLR with classic manual controls, maybe even in a retro-styled metal body. Include a fully auto mode of course, but make the manual controls the most important aspect of the body.
Second, I'd like a DSLR which is actually a scriptable and progammable applications platform. Pentax could define a set of APIs for third party application developers, create a web store for selling apps for the camera, things that could modify and extend the functionality of the camera in all sorts of ways. The camera could also come with scripting functionality so that anyone could easily modify the behaviour in simple but powerful ways, and those scripts could be loaded and unloaded to the camera on the fly (USB, Bluetooth or perhaps WiFi). The camera could also go into a mode where it could be almost entirely controlled by a computer, both by software Pentax could ship with it or with third party (or even entirely custom) applications. Pentax running the app store would mean they'd get a cut out of each app sold (just like the iPhone/iPad) and they could ensure a certain baseline quailty of the apps in the store. Perhaps the camera could also contain an expansion slot for additional expansion hardware, like a GPS or other sensors, maybe the camera would come with a multi-axis motion sensor and that info could be available to apps on the camera so that programs could dynamically adjust how the camera behaves based on how fast the camera is moving and in what direction while it is focusing and taking pictures. Maybe the camera would be powerful enough to do in-body corrections for lenses beyond just Pentax lenses, maybe custom lens corrections each particular lens a photographer owns could be possible. Maybe new artistic in-body camera modes would be possible. I have so many ideas on this, I could write for hours, but I have a full-time job and it is not camera designer... but there are people out there who would love this, just look at what people have done with the
Canon Hack Development Kit (CHDK) and that's not official!
Anyway just a couple of ideas I'd love to see Pentax make into real products some day.