Middle of the Market (MOM) airliner

Boeing is talking about doing a new airliner to be a MOM airliner.  Airbus is competing furiously, and Boeing wants a magic product to take market share away from Airbus.  Unfortunately, just what the MOM airliner might be is vague, they don't talk about how many passengers it would carry, or the range it could fly.  And some people feel there is no such MOM design.
   Obviously Boeing is still feeling good about their new 787, which although smaller than the Airbus A380, is selling better.  When they started the 787 they knew that Airbus was doing something much bigger, but Boeing figured that the 787 was about the right size and would sell better, and they were right. 
   The other thing that clouds the issue is that Boeing makes some many different sizes of airliners already that you would think one of them would be the MOM airliner.  They have the smallish single aisle 737 which is still selling every one that comes off the production line.  They have the 757 and 767 models, larger than the 737 and maybe to be dropped.  They have the brand new sizable 787,  the older but large 777, and finally the big old 747.  They are still making a few 747's but it is clearly on the way out.  Given this wealth of Boeing airliner types, it is hard to see a market segment for which they don't have a product. 
   For future growth, Boeing has the 737MAX project to put new and more efficient engines on the 737.  This project is going head to head with a similar project at Airbus putting the new Pratt & Whitney geared turbofan engine onto the tried and true A320 airliner.  Boeing has the 777-X project to create an updated version of the big 777 twinjet.  They have the USAF tanker project inhouse which something like 200 aircraft. 
   There has got to be some pressure inside Boeing to do another clean sheet design, using carbon fiber structure, and the latest of everything to create a follow on to the 737.  But the last clean sheet design, the 787, encountered delays, supply chain hangups, cost over runs, battery fires, and it's gonna take years and years of production to recover the money sunk into it.  The 787 has made it thru the development pitfalls and is now in production and making money.  But it was so late that Airbus was able to get the directly competitive A350 to market only a year after the 787.  Anyhow, there must be a lot of people at Boeing who have sworn "Never again" to the concept of advanced clean sheet designs.