The best resolve is not increasing AP cost but reducing effectiveness of actions individually for each class. This allow all skills to still be equipped allowing class customization and class uniqueness at the same time.

Example:
1. Cure Effectiveness:
100% CON
75% THM
50% DoW

2. Defender Effectiveness:
100% MRD
75% DoW
50% DoM

These are just examples of how actions can be scaled individually so that uniqueness is increased. SE can change these values to whatever they want. Although this idea is implemented much more effectively with the Title System rather than based on class. (Link #1 in my signature.)