Basically once we unlock a job the class becomes pretty much obsolete. If nothing else because of the pure stat boost that comes from equipping a soul crystal. Not to mention the relic weapon and usually far more powerful skills.

So my suggestion is to add something for classes to level the playing field. Soul stones. Basically it will go in the soul crystal slot but won't change your job. It will have enhanced stats on it that you can use to customise your stats to suit how you want to play.

For example:
Soul stone of power: +10 STR, +10 DEX, +10 VIT, +3 weapon damage.
Soul stone of knowledge: +10 INT, +10 Piety, +5 magic damage.
Soul stone of wisdom: +10 MND, +10 Piety, +5 magic damage DoW: +20 magic damage.