I generally like this idea. But I don't think Job Abilities are the best for this.

Cross-class abilities are currently a crutch to provide identical skills and mechanics between Jobs and Classes of the same role. It was built because of legacy 1.0 coding and is retained because of gameplay. However, I don't think Job Abilities should be used as cross-class and should be unique to the Job. You would have to recode Job cross-classes anyway because you need a more restrictive version for cross-class anyway.

Job Abilities are designed to fit within the theme of the Job and to fit together to provide a cohesive gameplay experience. If you let Paladins cross-class something like Tetragammation or Benediction, it won't necessarily make them better tanks. It makes them better healers (which may cause a change in the meta, but that's not the point of this thread). The point is that Job Abilities carry a lot of luggage in terms of flavor, lore, and gameplay that won't necessarily mesh well.

Instead, I want a pool of general abilities that all Jobs and Classes can use. Some Abilities may require certain class or job levels to unlock, but all of them can be accessed by any Job/Class. The benefits of these abilities are going to be slightly less powerful or have longer CDs than those earned by Jobs or Classes, but that frees up cross-class abilities for actual customization. Roles may have their own sets that come gratis (again some may require unlocks) that don't take up a cross-class slot (Provoke for tanks, etc). It'll require balancing yes, but so would any attempt at creating alternate versions of existing Job abilities.