1) Start with all pieces off the board. To make a move, you can either put a piece onto your half of the board, or move an existing pieces. Captured pieces cannot be reused (or maybe they can, if you want some sort of bughouse variant).
2) Put a sticker under one pawn on each side. These are the 'assassins'. Be sure to randomise your pawns without looking at their bases. During the game, as a move, you may turn over one of your opponent's pawns - if it is the assassin, you may choose to reveal it. If you do, you now control that piece as if it's your colour and may move it immediately. This of course means you have a 1/4 chance of the opposing king being in check, in which case you can capture it and end the game right there.
3) Ghost pieces. Whenever a piece is captured, remember the square on which it died - it now haunts this square. Any time during the game, a ghost piece may make any capturing move from its square, but instead of capturing the opposing piece, it merely frightens it. Frightened pieces can't move during your opponent's next turn.
4) Tunnels! Cut out some cardboard or paper in a randomish shape to overlay any number of squares in the middle of the board. This represents rock, into which pieces cannot move. The first move a piece makes into the rock weakens it. The second clears the rock out of the way (keep a pair of scissors handy).
Some of this seemed like a good idea at the time.