Some of my friends and I made a variant where, before the game, a piece on both sides of roughly similar value (but not the queen or king) was designated the traitor. However, neither side knows which pieces are traitorous. Play starts as normal, but after a player has just a quarter of their time left, he learns which of his opponent's pieces he controls. He does not learn which of his pieces is the traitor for his opponent. Play continues, and on his turn a player can, whenever convenient, reveal the traitor and use it as his own piece, or he can negate the previous player's move (if the traitorous piece made the move) and make a different one instead.
It was pretty awesome. There all kinds of strategies, such as trading everything or trading nothing, leaving your king less protected, or sacrificing three quarters of your time to learn which piece is the traitor you control. It's a little nerve-wracking leaving your queen somewhere your own knight could get to it, and if your opponent doesn't announce a traitor quickly, you'll not know if it was traded off earlier or if she's just holding it in reserve. Tons of strategies and tons of fun!! I highly recommend it!