Okay so we're working with TP now as well, great
I'm also thinking we could add very odd elements.
For example: Oil.
Oil and fire would be explosive, so very damaging.
Oil and water would separate, very little damage.
Oil cleans metal, increasing metal damage.
Oil prevents flight in winged creatures.
Oil makes enemies and players harder to hit, but also less likely to hit their target.
So now you're fighting a mage, and he keeps using water attacks, so you pull up a fire shield and he hits you with oil.
But if you put up an oil shield, maybe them mage can't cast fire attacks, so you'll be safe from the water.
So you base your strategy around what attacks your enemy can and can't use, as well as what you can and can't use.