Here is the idea. When you open your chests there is an extreamly small chance that it turns out to be a Mimic. When you open it a battle is triggered with the mimic. If you beat the mimic you are rewarded with a special card that can only be obtained by beating the mimic.
The first special card should of course be a mimic.
I would have it be a Legendary 0 mana card that has backfire and scavange
earning shatter at later levels.