Give BLM 3 elemental stances so they can change their main damage focus based on the situation.
Weak to Ice? Use "Ice Stance". Now the blizzard line deals higher damage and can combo while thunder and fire get crowd control properties and thunder loses the damage capabilities. "Fire Stance"> Fire spells are now single target with a different combo bonus, thunder and blizzard for crowd control. "Thunder Stance"> what we have now.