There has long been discussions about a new mid-market sized aircraft from Boeing, and it has long been decided against. However, it keeps coming back as an idea as there is demand from airlines for something bigger than a 737 and smaller than a 787. The largest 737 models are unpopular as they are seen by many operators as a stretch too far for the base aircraft - and the advantages of a larger base platform, like the A320 and 757 become more and more important. For example, one of the key reasons that BA disposed of all their 737's to concentrate on the A32x series was to allow them to move to containerised baggage - something that is not possible in the 737 design. Likewise, the smallest 787 models are a lot of aircraft - and hence a lot of money - and optimised for long haul routes.
That said, they keep deciding against a new model, as the market isn't particularly big for an aircraft that size, and the 737 / 787 are seen as just about good enough.