Even better. You should have 5 choices at promotion. If you choose the 5th option, a King, the King is allowed to move into check, but can't be captured. If both Kings are in check, you must move one of them (or capture or interpose). In order to win against an opponent with 2 kings, you must checkmate both Kings. The only form of check that a King can't walk into is check by the opponent's King. So now, when you promote, do you try to win by promoting to Queen, or do you take an almost sure half point by promoting to King?
Major rule change

When the king eats an ennemy piece, he takes the power of that piece. Ex: after Ke1 x Nf2, the king now moves as a knight!... Or Ke1 x Qd1, now it's a super-King!!!

At a tournament, instead of moving you can reach over to the player on your left, grab a piece that has been captured of your color, and place it anywhere on the board.

At any time, you can flick your queen to take out any other pieces in the way. All that fall over are immediatly "Dead". (The flicked queen dies as well)

how bout if a pawn captures a piece it becomes that piece.
There is a game called plunder chess that uses that idea. It's worth a look. Basically if a bishop took a knight, it would have the ability to move as a knight for one turn.

vodka-chess rules:
[there can be variations]
whenever you take a pawn, you drink 1 shot of vodka
whenever you take a bishop or knight, you drink 3 shots
whenever you take a rook, 5 shots
whenever you take a queen, 10 shots
you have 1min/shot

Have the queen gain the knight's moves as well. The queen already has the rook's and bishop's moves. Another rule would be that you have to capture the opponent's king to win. So if you checkmate an opponent, they can still make a move, then on the next move you capture the king and you win. Also, if your opponent fails to move their king out of check, on the next move you capture their king and you win!

The queen gains the pawn move, so it can move one square forward and capture on square diagonally. Also it gets to capture en passant.
When the queen reaches the 8th rank, it becomes a pawn.
B oth players are allowed to secretly choose 1 square on the board where if any opponent piece lands there that piece is captured. Once it occurs a new secret square is chosen and the previous square returns to normal.
These squares will be called MODs - minefields of death.