Coming from a photographer: stacked sensor has nothing to do with improved dynamic range or capturing more light.
Stacked sensors have faster readout speed, meaning they have less rolling shutter. This was basically never an issue with smartphone cameras, so I'm not sure what benefit it can provide, maybe they will spin it up with some marketing terms. In order for them to capture more light they need to use a larger sensor, or a brighter lens there is no other way.
Yes, given how the camera seems to be the bulk of the "upgrade" rationale almost every year now, and the laws of physics are more rigid than Apple's walled garden, I find myself thinking this: instead of continuing to try to glom on an ever-improved camera to a phone form factor, how about going the other way? What if Apple would create a new camera product that gloms the other iPhone benefits onto a camera form factor?
All those rigid physics inhibitors would be eliminated because a relatively skinny brick would no longer dictate all. And frankly, the bulk of the non-camera parts will work just fine on a screen on the back of this hypothetical new product creation by Apple. Wear it around the neck like photographers have for many decades...
In this case, this hypothetical Apple Camera would also have all of the non-camera, iPhone benefits too. Use buds with mic for calls (as many already do).
In my case, I opt for iPad Mini (Voip app and buds) to double as my phone and it works just fine as a phone. At times, I take or make calls on my MB. For something to be a "phone," it doesn't have to be a roughly hand-sized, skinny brick. Telephony is just an app... like Maps, Flashlight, iPod, Tape Measure, Compass, etc.
A true camera could easily do that too. Put the same size touch screen on the back of this camera which would double as a big viewfinder (as many do with their phone now).
Obviously, this would NOT replace the brick in our pocket option... just offer an alternative for those who care much more about the camera than the "rest." Premium cameras sell for Apple-type pricing & margin. A hybrid Apple-branded camera that also has all of the app benefits of iOS seems like it would be quite desirable to that crowd vs. buying only someone else's full-sized cameras alone.