Metal's opposite should be plasma, and vice versa. And then Plasma's weakness should probably be fire or light. Most likely light though.

Air's weakness should probably be Earth and then since you've already established that Dark has power over Metal then Metal's weakness should be Dark. And so Light has power over Dark. So the ones that still require weaknesses/power overs are Plasma, Light, and Metal.

Okay, I kind of just threw stuff together at the end there. If you think something would work better please tell me, my brain isn't working right now.

Also, do you think I should alter anything about what the powers can do?

I actually think you should perhaps add more to Earth, Air, Fire, and maybe Dark- The others seem developed enough but especially Earth, Air, and Fire seem a bit underpowered when compared.

I agree with that, any ideas about what I can add?

Perhaps some form of telekinesis for Air... Not too sure about Earth, Fire, or Dark though.

I have blogs about magic that you could look at though.

Okay, I added more to fire, air, and earth, how do they look now? I still couldn't think of anything for darkness though...

You could give them echolocation- That would be pretty cool and fun... Or perhaps something like seeing into the past, since Light is it's opposite and they can see into the future. Or perhaps they can speak to shadows.