I don't see a gap in camera-processing for either phones or DSLRs, because the experience sucks on both. But you are right there is a gap between "consumer" software and "pro" software that could be addressed.
I don't know if offloading a bit of processing to your phone from a camera would fix much really. Ergonomics of cameras is much better for taking photos, and there isn't much in there you don't need.
I guess you could get rid of the screen entirely, and some of the user controls. That could be nice but would be difficult from a product point of view, as you would be dependent on the whims of a larger market. See all the difficulties getting standardized on things for car, where there is much more incentive...
I'm hoping to hit a chunk of that "middle option" you spoke of with PhotoStructure (details in my profile). I'm building sharing next, so hopefully that aspect will be addressed as well. Hosting the software yourself addresses privacy concerns. The beta is being offered for free in exchange for feedback.
Anyway, I would have thought the dealbreaking issues with an augmented phone/camera would more be around the dearth of tactile controls, but that can be at least partially addressed by moving some of them to the strap-on accessory.