Airbus is pre-marketing a new concept, a 400 seat stretched version of the A350, tentatively dubbed the A350-8000. This new model would compete directly with the 406 seat 777-9. According to John Leahy, the new model would have similar capacity and range as the 777-9 with substantially lower seat-mile costs.
Our analysis of the Super Twin market in 2013 projected that it would become a competitive necessity for Airbus to match the 777-9 in seating capacity to more effectively compete in this market. It appears that Airbus is now moving aggressively in that direction.