I would expect it would cost more than just the price of a regular MBP so it wouldn't be targeting the 'every user' base. But it would be a great option for those who want both a notebook and an iPad.
As I understand it, the current MBP screen only contains the screen plus camera/mic/ambient sensor. So replacing the screen with an iPad would cover all those bases. The iPad would still have its own battery, memory, processor - everything it already has. But when it is docked via the smart connector and some kind of hinge, it would enter a 'display mode' that would turn off or balance those other components as necessary.
The MBP base already has its processor, battery, memory, etc. so all of that would stay in there. Implement some battery balancing between the two - similar to the iPhone battery case so that the iPad is ready to go whenever you want it.
I'm sure it would be thicker than a current MBP because of the extra heft of the iPad, but I'd be OK with that.
Do you know where children get all of their energy? - They suck it right out of their parents!