What if casters had element resistance debuffs?

[Joe777]

That would only be effective for BLMs, RDMs, and DPSing WHMs, since other magic is classified as unaspected. It would probably be cumbersome to have to change the effects each time you switched to a different type of magic. BLM would probably be the only one that would benefit from them for an extended period of time, given how RDM rotation swaps between different Ver spells, and SMN is completely unaspected. Same for SCH and AST.

A better alternative to something like this would be just a sweeping "magic resistance down" debuff.

Actually some melee jobs have "magic" elemental attacks too, especially Ninja. Bard has it too in attacks such as wind/stormbite.