Basically, these cards would be a permanent. Whenever "x" happens, "y" happens. More powerful ones could have a single use effect.
For instance:
When your opponent plays a creature, you draw one card. Negate this effect if you hold seven cards.
When your opponent activates an ability, you gain two health.
When a creature dies, poison one of your opponent's creatures.
When your health is below 20, all of your creatures gain +2/+2.
The list could go on and on, but I think yall get it by now :-p It would open up an entirely new area of tactical play.