I have lots of hours in the 767 and love it, but an exec at United 25 years ago said: "We want more seats, not more aisles." A narrow body is 6 across, with 1 aisle. The 767 is 7 across, and 2 aisles. You get a airplane 40% bigger and heavier, for only 15% more seat capacity. Not a good trade off.
I think the MOM will be a long narrow body. Maybe Boeing will "dumb it down" with a smaller wing, engines, and landing gear for a modern narrow body replacement.
Fuselage in 3 lengths, two different wings and engines for different missions.
And it won't sell very well because the 321 will already own the market, and they can sell it for half the price as the new Boeing.
Boeing dug themselves a 737 hole and are now stuck in it.