Some elements already weaken each other.
Fire:
"el_damage": {"fire": 0.5, "ice": -0.5},
"el_defence": {"fire": 0.2, "wind": 0.3, "water": -0.6},
Ice:
"el_damage": {"ice": 0.5, "fire": -0.5},
"el_defence": {"ice": 0.2, "water": 0.3, "earth": -0.6},
Thus if you have both elements, Fire and Ice, which are opposite to each other, your spells will get zero damage bonus. You will get good elemental defenses though, but also weakness to both water and earth.
Well, at least if the system works like it's supposed to
Basically, every element will give you 0.2 and 0.3 defenses to two other elements, but takes away 0.6 defense from a third element defense. You don't want too many elements because they will take more defense than give.
I updated my small "patch" for Cherry's naruto pack, it now has pics for two other characters.