One variant could be made in this way.
1-The pieces values of all enemy pieces he captured, cant be higher than 21 (promoted pawn pieces count as the value of the piece pawn promoted to). If this value gets higher than 21 the player lose.
2-When one player mate the enemy king, if value of the pieces he captured is higher than the total value of pieces the other player captured, he wins, if the value is smaller than the other player value or is higher than 21 he lose.
3-In the case of a stalemate and draw, the player with the highest amount of points wins.