not an aircraft designer from Boeing Commercial Airplanes, but in my opinion they should have MAXimized the 757.
There are thinking on that, There's article from Reuters that saying Boeing considering 757 Plus concept. Miss that article, however found this video that talking on that concept.
Problem are, Boeing close 757 line too soon. They're seems miss the market trend that moving to Cities Business model more instead Big Hub model. Thus building 757 replacement base on 757 it self in theory should be faster. However since they're already closing the production line, then they have to build it from scratch. Thus it's not much difference then building completely new design.
would have thought that they would go for a B757 replacement which is something that I think that the market wants. One thing that I sort of sense since the McDonnell Douglas cabal takeover of Boeing management, is the arrogance that set in and apart from the B787 the apparent lack of a new clean design product.
Based on several media articles toward the years, what seems already cleared for this Middle of the Market segment is that the market wants Airliners between 220-270 capacity with the range of 5000+ miles, and have economics of operation more comparable to single aisle rather large twin aisle.
There're concepts on how to achieve that, just like what Merry Lynch talk with Boeing to create this NMA from 737 based but fatter thus can be fit with 2-3-2 configuration. The idea is to utilize 737 line thus reduce the costs.
However we have seen what happen on stretching the 737 design too far. We see the results on MAX. Thus I don't think Boeing will move on that path again.
There's the idea on using 767 or downsize 787 just like @John Fedup
put. Still I suspect using 767 or 787 as based will not provide the economics of operation that the market ask. It will not be able to compete with A321 XLR economics.
Personally I like the idea of 757 Plus, rather than build small size twin aisle. Yes it will still be costly to reopen 757 line. However considering Cities to Cities business model that taking on, also after COVID I suspect many passanger want to go direct and minimising transfer in Big Hub.
Thus who knows, perhaps they're thinking on building new Airframes based on the Airframes being replaced, is more acceptable for the Market.
I think right now, perhaps some Boeing Executives smack their face and say; "why we close 757 line to begin with".