For the element specific masteries of the magi, I believe some restrictions should be set here.
What I suggest, is the magi is allowed a primary element. Then, they are allowed a secondary element. When they choose their primary element, they are not allowed to choose the opposing element for their secondary.
For example, if Magi Matt decides he wants to take on the element of Fire as his primary element, he is not allowed to choose powers from the Water element for his secondary.
If Magi Matt decides to take on Water as his primary, he can only take on Earth or Air as his secondary, and not fire.
And then, obviously, Earth and Air would oppose each other. Ideas?
That was the original idea behind them, though I just cut it to the same as the Psions and decided that all other disciplines/elements will just cost an additional 25 XP, since we will have Earth, Air, Fire, Water, and Spirit, and Spirit is tought to fit in there with an opposite.