The A321neo is mainly limited by its wing, which is highly loaded. But there are ways around this limitation other than developing a new wing, an exercise which would require time, money and a new certification program.
Having understood the limitations, we now look into what can be done about them.
The changes to the wings can be limited to those that are easy to do, resulting in longer range.
Combined with fuel capacity improvements and stronger engines it produces a very interesting aircraft.