Boeing studies may conclude that at the moment an ideal m.o.m aircraft is something with say, 20 more seats and 200 nm greater range than the A321LR. However developing this aircraft would cost $X billion dollars and would have to sell for $Y million, airlines are unlikely to want to pay a huge premium for a small increase in capability.
A massive investment can only be justified if the aircraft sells in sufficient numbers, so it would only make sense if Boeing’s new m.o.m aircraft was common to a B737 replacement family where the development costs could be spread out over the entire range. A single isle 180-190 seater from Airbus or Boeing is going to sell in huge numbers so substantial investment is justified and having a small improvement in fuel burn or an extra 200nm range could be the determining factor in a 200 aircraft order.
The B737 MAX is a stop gap until a clean sheet replacement can be developed. Pumped up, blinged out and sold cheap, to hold the line against Airbus while new designs take shape.
A similar approach could be taken with the B757 airframe. Winglets, 787 flight deck and current generation smaller engines would give an aircraft that would cost very little to develop as it uses currently available technology and could easily be made slightly more capable than the A321LR giving Boeing the lead.
The B757 was overpowered and not all operators needed or used its full capabilities. Smaller modern engines could power a new version, which whilst not giving it the payload/range of the old one, could still do the job and offer a greatly reduced fuel burn. The opportunity is there to tailor the aircraft to specific requirements in terms of take off weight, payload/range and fit appropriate, currently available engines.
This becomes Boeing’s m.o.m aircraft until the new range of narrow bodies becomes available in 10 years time. It slots neatly into the gap between the B737 MAX and the B787-8, costs little to develop and can therefore be sold cheap. As it uses a 1980s rather than a 1950s airframe it can incorporate modern technology. I find it unusual that Boeing have already retired the relatively modern B757 fuselage but continue with the antique B737 version to this day.
It would plug a big hole in Boeing’s range and could retain customers who were considering moving to Airbus.