If Boeing were looking here for suggestions (ha!), mine would be to bring back and re-engine the 757. That airframe should have a lot more potential for future development. Of course, that would mean that the Southwest pilots would all have to be (re!) trained....
One other factor: Southwest essentially owns William P. Hobby Airport near my home in Houston. Due to its 1930s design and tight taxiway clearances, that airport is closed to any aircraft with a wingspan greater than 125 feet. The wingspan of the B-757, as previously built, was 124 feet 10 inches. Doesn't leave much room to play around for aerodynamic tricks.