[duelingjello]

The MAX was a "bridge too far" for the 737 type. They need a clean-sheet design with big engines and fuel-efficient wings to compete with the A320neo, and the 737 can't be adapted to do that (wings too low to the ground) and it has a bad reputation. They're better off developing the Future Small Airplane (FSA), and call it a "797."

[tropo]

Boeing can build a fuel-efficient aircraft that is highly compatible with the 737 airport equipment. There are two obvious approaches.

The first approach is that taken by the Boeing 717. Do a T tail with rear engines. Bombardier, Gulfstream, and Embraer commonly do this.

The second approach is that taken by the Boeing C-17. Mount the wings high. Although usually done for turboprops and military cargo, passenger jets have been done in this configuration. The British Aerospace 146 is an example.

There are many more unusual options as well.