I don't think they should label enemies like that. Having enemies reveal their type w/ their first attack would be one way to "equalize" the initiative roll.
Anyway, some other ideas:
(O) "Big" spells that take time to cast--i.e., your character would spend a full turn "casting." This would make the character extremely vulnerable to elemental damage of the opposite type. I.E., if you're casting a "meteor" (fire) spell, you'd take 200% damage from water-based spells. Something like that.
(O) Inherent Elemental Weaknesses. For simple units who can only cast a single element of Source Magic, it would be simple. Something like taking 150% damage from opposite-element attacks. But for special units (PCs and special--boss?--enemies) your elemental weakness would shift depending on what spells you cast. So, for example, if you cast a fire spell, you will be especially vulnerable to water-based attacks UNTIL you cast a non-fire spell, in which case you would then be vulnerable to whatever its opposing element is.
This has some nice potential to incentivize advanced tactics. For example, if you're fighting against a fire mage, you could do extra damage with a water spell... but casting a water spell would leave you vulnerable to recieving extra damage, yourself. To make your magic count, you'd have to try using complementary spells to increase the effect of your fire magics. For example, you could cast a "summon wind" spell (that would increase the area-of-effect/damage/duration of fire magic) and an "earth wall" spell to shield your caster from enemy magics before unleasing your fireball. Things like that.
(O) This is kind of tied to the above... but if the magic system is going to revolve around using opposing elements, there should also be complementary elements. I've touched on this before, I think (w/ the dual-casting idea). Basically, you'd be able to increase the elemental power of Y-element magics by using X-element magic. Things like that.
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Well, I had some other ideas, but I've actually gotta run out now, so I'll post 'em later if I can remember 'em.
