The only real "class gauge" I feel would work is if they made the elemental weaknesses thing the actual BLU class mechanic. Meaning extra damage for actually using weaknesses and whatnot in normal dungeon/raid/trial/OW content. Example, using the ice/water spells against Ifrit would 1.5x or 2x the damage, fire wouldn't do anything, earth would do normal damage, etc. That makes all the spells much more valuable and gets players to mix up their kits for content simply because they have to in order to optimize damage.