How about a small stretch of the A380, primarily to increase freight capacity, but also to keep engineers busy in the short term.
The A320 family is probably coming to the end of its stretch life, and airlines are cramming ever more seats into the available volume. Is it time to start thinking about a completely new shorthaul offering? 200+ seats at reasonable pitch, twin aisle to make for faster boarding, seven or perhaps eight across in Economy. Would it sell?