There is a clear gap in the Airbus product range between the A321 and the A330. It used to be filled by the A310, but despite its popularity (whenever they come up on the used a/c market, they're snapped up immediately), Airbus ave never updated it...
ooo - perhaps an 'A325'? Same size, same wide-body twin-aisle layout as the A310 but with a FBW and full-glass makeover?
Thats the reason boeing are making the 787-3.
Incredibly, with the A350, Airbus have again ignored the need for a plane like this! Instead they've gone chasing after EK who stated they needed something bigger.
Future 737NG pic(?)
The A321 is the limit of the A320 fuselage and wing design. The A321 is already borderline on the economics.