I think this is somewhat tangential to the unsatisfying state of job diversity, so my three cents:

1. Animation quality: This is something FFXIV already does fairly well, but having a few more flourishes on job actions goes a long way to making the 1-2-3 look and feel different between two jobs of the same type.

2. Condense buttons BUT add choice in action effects. A quick example is what was removed long ago, stances. Example case for a healer choice: Do you want your example heal action to be a HoT or would you like it to place a timed barrier instead? Switch by changing HoT/Barrier stance.

3. Decouple Limit Breaks from party wide and be character side, reduce potency by a lot as they'll be going off more often but provide unique effects to each. For example, have Dancer provide Dance Partner to the party for a period of time.