Just an idea I came up with: Negating the effect of some (non-token based) keyword with another. Most obvious example that comes to mind is Restoration and Destruction. For instance;
Player 1 plays Uncommon Destruction card
Player 2 plays a Restoration card (of X rarity)
Player 1 can no longer trigger the Destruction keyword this turn. He could, of course, play another Destruction card and try again next turn.
It would work the same in reverse, so Destruction also counters Restoration. Other keywords such as Illusion, Nature, Horde etc. are a bit less obvious and need more thinking to set up.
Or it could also be a whole new keyword that interrupts some keyword effects (or a reworked, existing one).