IMHO- rather than stretch a 737 or make a new version 757- both single isle- an updated and slightly stretched 767 twin isle would be a major game changer. Wings of composite, a bit bigger engines can be added without the 737 limited clearance issues, and a few other tweaks- updated cockpit- enhanced fly by wire, etc could fill the ' passenger seat gap '
But since I'm not a jack swelch wannabee- or a mdc defense convert used to govt tit- I'm obviously wrong- power point rangers rule . .