Many years ago, I heard that Boeing was going to whittle their fleet down to the 787 for their international jet, and a narrow body domestic version of the 787, (a plastic 737) same systems, same composite structure, etc. to replace the 737.

But that was before they lost their arse on this 787 debacle, first with being 3 years late to deliver, then by parking the entire fleet with the battery issues. I'm guessing they aren't going to spend a whole lot of money developing another new airframe, be it a narrowbody 757/737 replacement, (797?) when they are still selling 737's.