Yeah or rather the core idea.
And the Black Mage idea has not been "refill mana in ice phase to blast huge damage spells in fire phase" for a long time. That's just not enough to sell a class on in an interesting way. It could be, I suppose, an in that case giving that mechanic early could work. For example mana regen in ice phase could be non-instant (you get some for each spell used while in ice, and nothing costs mana), but all spells are instantaneous to cast in it. Meanwhile in fire phase everything has a cast time (no instant spells!), Triplecast is removed, too. But swapping is an oGCD and Despair could drain all mana and swap to ice meanwhile Manafont is an ice-only ability that instantly restores all mana and swap to fire or so. Meaning your movement is for ice, and standing still is fire, and you want to keep your resource floating in the middle instead of capping or depleting it, ever. And all potencies rebalanced. (not a truly serious idea, just to explain that the base ice-vs-fire could be inherently an interesting mechanic)
It's like, Astro gets their astral/umbral card cycle pretty early. Sage sadly does not get shields giving sting early, which is ultimately the main flow of the job. That's where they ought to focus, have each job have the core crucial gameplay pillar early. Dragoons start lvl1 lying dead on the floor etc.