>I will say I'm slightly surprised we haven't see an external "lens and sensor only" gadget that pairs with a phone.

This, camera tech on phones is great, but it's never better than a dedicated device. Plus, the models with best cameras aren't always the best phone experience. An external camera device that you could attach lenses and such to, but could dump the data on to your phone, and get GPS and such from your phone, would be a great middle ground.