From Leeham News and Comment

Boeing is quietly studying how to improve the 737 Next Generation, according to sources quizzed for a report in the December/ January issue of Aviation and the Environment, who all have knowledge on some level of the Boeing studies.

Among the possibilities being explored is the placement of a scaled-up version of a Pratt & Whitney geared turbofan engine on the aircraft. This would necessitate a massive structural change of the 737, with the introduction of a new wing, taller landing gear and a redesigned wingbox.

Additionally, a new avionics system seems likely, in which RNP would come as standard. A range of internal systems improvements are being considered, which would make building and maintaining the aircraft easier.

The combined fuel burn improvement of these measures could be as much as ten per cent - potentially making it very attractive to airlines as a “bridge” aircraft…

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Is it really worth going to all the expense of a "massive structural change?

It's time for a new Boeing single aisle. The 737 is too narrow for comfort. It's also too noisy.