This card is a great idea, no matter what wording you choose or what color you make it. It would definitely give a reason for creature decks to be careful, and no, I don't think it would change the basic game at all...unless your idea of the game is entirely dependent on creatures.
Sure, many creatures are designed to sit on your front lines and spam their ability 'til doomsday, and true, creatures cannot be ignored in this game. But really, is adding this card concept so bad? The point of the game, to me, is to challenge dominant deck strategies and shifting the balance of power whenever it gets out of hand. Can we not agree that activated creature abilities could use something to fear? Especially in the PvP department, AI might need coding to notice this card.