Whoa, that is a cool idea. But I think it has a few issues. First, your right-hand opponent can foil it by building an Action-less deck, which sucks. What if it also let you play Treasures? Second, it's an unfortunate timing for revealing a Reaction. Most players just scoop up their cards when their turn is over. You have to say, "Wait! I want that card!"
Maybe if it were an Action that allowed you to look through that player's discard pile and play a card from it? Of course then you could do it after they gain the card. Also, it loses the charm of being a pure Reaction. Hmmm…