I think that Boeing is deciding what direction to go with its narrow body offering. The 737 Max is a stopgap. The 900, by all accounts, is a smasmorsion. I bet they are going to design a new narrowbody airplane based on 787 design features to maximize range and minimize fuel consumption. It won't be a "ground up design", but a spiral evolution on existing designs. The 787 was a huge evolutionary leap. This would be the love child of the 737 and 787.
Boeing is at a huge crossroads and needs to take the next step. The 747 is winding down, leaving a huge hole in production capacity. The 737 is tired and the 900 is doing a poor job of filling the gap left by the 757. As the last of the used 757s get snatched up, I think we are going to see a need for an aircraft with the same range and better fuel consumption. Not every airline in the world wants to fly wide bodies and fuel cost is going to be a huge driving factor from now until eternity.