A321LR will be the game changer as it will enable low cost airlines to try longer routes without a huge commitment. A city pair can be tested and quickly dropped if it doesn't work out. The aircraft then returns to normal short haul service.
No need to go out and get a new fleet of B787 sized aircraft with the high purchase or lease cost and set up expenses as a A321LR will do a lot of what a B787 will as long as you don't need 300 seats and 10 hours range.
Low cost has so far been limited by the non stop range of the A320/B737, as the circle increase more and more people come into the catchment area. Beyond a certain point an out and back won't be possible and the crew will need to overnight, but if it still works out then why not ?