I don't know if they are rumors, but I hear that Lockheed's QSST proposal is not using standard jet engines, that the combustion chamber is pulse-based, meaning they are not burning fuel all the time. This could translate into more range, more payload or less fuel costs.
I wonder if Boeing got a hold of that engine and derated it to subsonic flight, would 737RS rape 787 in terms of fuel efficiency? Think about it, if an engine was made to fly supersonic with much less fuel usage than previous generations, imagine what a subsonic variant could do...transition would be similar from SC to 787. Even changing the engines on 738/739's could be a 757 replacement in both range and payload (maybe just frieght not pax). This type of engine could allow for longhaul LCC's.
Or go the other way and have LCC's save almost half in fuel costs, the plane would just be lighter, maybe even add access to more airports. Possibilites are endless right?