It is a good idea, but how could it be implemented for each class?

BLM: maybe being able to spec between Thunder, Ice and Fire, being able to increase the potencies of only one within each tier. (You actually could be an 'ice mage', your rotation probably wouldn't really change, except more of your damage would come from your blizzard spells than a 'fire mage')

WHM: similar elemental affinity, with Water increasing your HoT potencies, and the other two increasing your damaging spells potencies.

DRK: being able to invest in either yourself, or your dark side.
Dark side would increase the potency of DA effects, and improve MP regen, while investing in yourself would increase mitigation under Grit and Souleater HP regen.

NIN: choose a just a to invest in. Ten= boosted Katon/Huton, Chi= boosted Raiton/Doton, Jin= boosted Daiton/Suiton.

Just a few ideas for my main jobs.