Airbus has said an all new aircraft(minus engines) would only save 3%. The fuel savings all has to come from the engines, as a redesign is way too expense. Boeing is essentially stuck with the 737, they can't build a new frame unless the government gives them $15 billion dollars.
As far as engines go, the next step in efficiency is the udf engine, however, that's probably not going to happen due to noise limits. Notice how it's much louder than an old MD88 engine.