I'm a long time Boeing stockholder. Regarding the 737 replacement, I have a suggestion that is outside the box, if you will. Design the new 737 with 1 larger engine but keep 2 small engines inside the main frame of the aircraft, that would be deployed in event of main engine trouble, that could fly the plane in a slow flight condition for say 500 miles. I don't know if this feasible or not but I like to think outside the box.
Why on Earth, Mars or any other terrestrial planet or moon would anyone want to do that?
The 737 (as well as 757, 767, 777, 787, A320, A330, A350) propulsion concept is straightforward and simple. Two identical engines, one hanging off each week. Only two to be maintained, both with absolute commonality, both sitting in a comfortable and easily accessible position for maintenance, both in an intuitive position aerodynamically.
But this new idea. Now there are two different types of engine, the main big one and the two smaller backup ones. Commonality is lost. There are also three of course, which complicates things. As for position, where do you put the big one? It would be awkward. Hiding the backup ones in the fuselage would just take up space and it would also requiring moving parts - yes that's right moving parts - to deploy them.
It's madness. Sheer madness.