I do not think that they could improve much on the basic A320 design which they haven't already done on the NEO. Carbon fibre will not do much for a short to medium range aircraft and any advances in electronics and avionics do not require a new basic design. Maybe for the A321XLR you would want a slightly larger wingspan, to fit the extra fuel in the wing, but the benefits do not outweigh the costs.
The 737 design has two basic flaws compared to the A320: No space for a longer landing gear and no fly by wire.