The concept of silence would fit in dark really well.
A 20|0 dragon would have to cost 20 quantums, and if it did, I'd probably put it in Aether. You could call the card manaspike.
Destroying a random card could fit in any element really, some of them fit better than sound would.
The concept of sound waves fits better with water.
A weapon that deals that much damage and destroys the shield is kind of overpowered, and even then, I'd put it in gravity.
Those are some interesting ideas, but as the current elements have so few cards, adding another elements would just delay the games growth even more. If more elements are added, it's going to be a while before they are. But if they are, this one seems pretty awesome.
As for the opposite problem, perhaps have this element be thunder and have lightning be it's opposite (I know it's cheesy, but it's a thought).