I may be out to lunch, but I suspect that Boeing will eventually announce a carbon-fibre 787-style 737 replacement, and it will cover all current 737 and 757 missions. How?
At some point, Boeing will be able to deliver 300 or more 787s each year once it clears its huge learning curve hurdles. Isn't such a big stretch to design and build a new tube about 6-8 inches wider than the current 737 (and 757 and 727 and 707) tube - thus eliminating the A320 family width/comfort advantage.
I've heard that more fuel efficient engines generally require larger diameters, and the current 737 can't hold a much larger diameter engine due to its short-gear profile. Sits too low to the ground. So any vast improvement in fuel efficiency would require taller gear (not unlike the long-legged 757 gear). So a carbon-fiber 737 would necessarily have to sit on taller gear similar to the 757 gear. All A320 family planes already sit higher, so WN would have to adjust to taller aircraft anyway.
No doubt the 757 wing would need some work but it's not like inventing the wing all over again. Essentially, all new carbon-fibre future 737s would resemble 757s and thus, Boeing could design the 737 to seat 137-200 passengers with a single aisle (and those magical 18-18.5 inch coach seats) with a range up to 4,000 nm, enabling flights to Hawai'i and short TATL flights from the east coast plus, of course, all the other missions currently flown by 757s. If Boeing sees demand for 500-1000 long-range single aisle 757-replacements, it could certainly happen. I'll bet that DL, UA and AA will easily demand 500 of them alone to replace the current 757 fleet. And by 2020 or 2025, most 757s will be aged enough to warrant replacement.
Of course, all this hinges on the 787 delivering the fuel savings that Boeing has promised for the past few years. If the 787 is as efficient as advertised, I predict that a narrow-body 787 will quickly follow, rendering the re-engined aluminum 737 that AA demanded and ordered to be a non-starter paper airplane that will never be built. I don't see a future for a dual-aisle narrowbody that has been talked about on airliners.net recently. Never gonna happen, IMO.