I've seen some very interesting (and well-reasoned) posts here on the pros and cons of a new MoM aircraft, versus an improved "here and now." Enlightening for me were the concepts that it is a limited market (number of airframes), mostly for the Atlantic, and it entails a significant capital risk (and time) for a manufacturer.

Since Boeing has demonstrated since the 1930s the concept of "let's use these old parts on a new plane," (B-17/Stratoliner; B-29/Stratocruiser, 367-80/every narrowbody cockpit except the 757...), I come to the sad conclusion that the MoM will be:

The 737 MAX WETNESS. (MAX With Eternal Tanks Needing ETOPS Selectable Stores).

It will just be a 737 MAX with two underwing pylons. For Atlantic ETOPS, the airline can load an external drop-tank under each wing, just like a fighter (or B-50, B-47, and B-52). If you lose an engine, you pickle the tanks off. They will be "environmentally friendly" by having a parachute, and convenient pre-paid postage to be mailed back to the airline, when found.

For those short domestic hops, no tanks, OR, they can be used as "travel pods" to load low-revenue ticket holders' bags....or maybe standbys and jumpseaters.

It will come equipped with a hook and drag chute. Growth versions may adopt a conformal tank....

Boeing and SWA stock will rise immediately upon the announcement. Pilot and F/A morale will sag.