More balanced version of Kidnapper should have "Combo: Take an enemy minion and put it in your hand." So you are actually kidnapping the minion, any minion, and the balance is that you'd still have to pay it's cost (most likely the following turn) to get use out of it. Imagine Kidnapping a Ragnaros the Firelord or something...epic possibilities for an Epic level card.
I believe this would make this card at least a little nicer while not making it overpowered but at least letting the card see play even for a season.
I think that's still a crappier version of Cabal Shadow Priest.
More balanced version of Kidnapper should have "Combo: Take an enemy minion and put it in your hand." So you are actually kidnapping the minion, any minion, and the balance is that you'd still have to pay it's cost (most likely the following turn) to get use out of it. Imagine Kidnapping a Ragnaros the Firelord or something...epic possibilities for an Epic level card.
I think another interesting version would be
Battlecry: take control of an enemy minion with 3 or less attack and remove it's ability to attack. (ancient watcher card text, not 0 attack)
Combo: Enemy minion that is taken has taunt.
You're kidnapping the minion and using it as a meat shield :D
But I liked whiskey jack's idea too, even though it is not really kidnapping :D (more like convincing the minion to your cause).