While true that the wings on the A320 are already very dated, unlike actually the 737 which got completely new ones with the NG, there is of course the issue that it is pretty much impossible to increase the wingspan except if going for a folding wing design as Boeing does now on the 777X. Which of course would increase weight, complexity and maintenance requirements quite a bit, especially on an airplane that can and does something like 10 to 12 sectors a day. They went right up to the 36m maximum for a code C aircraft with the increase in wingspan that the sharklets provide (from 34,1 to 35,8m). They could increase the wing surface of course, but that has its downsides as well.
Will they do it? Well, they do think about a new narrow body aircraft anyway, but of course a different wing (and pleeeaaasee: an updated flight deck) could ensure continued production well into the next 10 to 20 years.