Well, the whole point of this is a surprise sneak attack. Making a new mechanic isn't bad in and of itself. Otherwise we'd run out of design space. Lots of the newer cards introduce new mechanics. Position based effects, stats based off of abilities, adding cards to your hand, etc.
Off the top of my head, what if we modify pounce to increase in power depending on how long it was stealthed? A static +5 with an extra +1 for every X turns stealthed?