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It seems that you did not unterstanded me very well. I wanted to know what is the sense behind the promotion of the pawn to an opponent piece. And as an alternative that the silver or gold are still useful when a pawn is promoted to one of them is not becoming an opponent piece but dropped as own piece on one of the free squares as like in Shogi. Ex: White promotes a pawn to a white silver and then dropping it immediately on one of the empty squares.

The only variance where your system works is when players are only allowed to promote to already taken pieces. If there is none available then they are enforced to promote the pawn into an oponent silver or gold. (imagine a pawn takes an opponent rook on the opponents base line and have then to be promoted to a opponents silver or gold.)

But interesting the idea you had with letting the transformed opponent silver or gold being dropped everywhere and not allowed to move next move. Cen nice be used to block a kings escape way.