Variations on the drop rule for crazyhouse chess (or shogi):
Pieces may only be dropped on the original position (two possible places for rook, knight, bishop, 1 place for queen, 8 for pawn).
Pieces may only be dropped on their original rank (1st or 2nd rank). Further possibility: some (maybe all) pieces start in hand, to be dropped on their original rank whenever the player feels it fit to do so.
Pieces may only be dropped on squares not attacked by the opponent.
Pieces may only be dropped on squares controlled (attacked) by the player. (Previously explored.) Further possibility: pieces may be dropped directly onto an opponent's piece, capturing it.