[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.