A "narrow-widebody" would fill the large cap from the current 737NG to the 7E7, but I still question the economics of an inheriently heavier aircraft and wonder if a wider fuselage would allow for <130 seat variants.
If we forget the twin isle bit for a moment and just look at fuselage widths, the 763 (5.03m cross-section) is being replaced by the composite 7e7-3/8 which have a cross section of 5.74m. Thats an increase of around 14% in fuselage width and takes seating from 7 abreast to a generous 8 abreast (9 abreast if required). The extra drag of the wider 7e7 fuselage is no doubt compensated by lighter weight along with superior aerodynamics and engines to produce a 20% lower fuel burn than the 767.
Translating this to the 737, a 14% increase in width gives an increase of around 20 inches (internal) in cabin width. Again, the extra drag would (hopefully) be compensated by lighter weight and superior aerodynamics to give a lower fuel burn than the existing 737.
These are all very approximate numbers - feel free to correct me if necessary.
The logic here is that if it works for the 767/7e7 then why not for the 737 - it seems that a 20 inch increase may be more appropriate than 30 (would shut the door to twin aisles).
The advantages of the wider cabin would be:
1. Far superior comfort than the competitors product.
2. The ability to stretch the design to just under 7e7-3 capacities (250 pax)
3. More suitable for IGW versions with longer range - upto 5000nm.
The wider fuselage would probably shut the door to variants <130 pax but this is getting into RJ
territory where even a composite shrink might find it hard going up against efficient RJ
designs which themselves have been stretched from 70 seaters to 90/110 seats.
The increase in fuselage width will have to provide benefits which offset any increase in drag etc. 30 inches, twin aisles and the like might be a step too far but 20 inches seems plausible given what has been done with the 7e7.
Again, I think Boeing will be determined to offer something that the 320 cant match.
Just my $0.02