I always thought it would be cool to have them work kind of like Ninja does, with their classic "Mix" command to execute and other mainstay ingredients like dark matter controlling what gets done. Like you'd have one "item" for HP healing, one for making it AoE, you'd press one of each of those and then hit Mix to execute their version of Medica.

Unfortunately, like other people pointed out, the devs frequently paint themselves into corners with the way the roles work in various aspects of the game - role actions, limit breaks, and weird role-specific stats like piety.