A piece (usually a pawn) can only promote to a piece type of the opponent's that has been captured. Three variations on the concept:

1. Promoting to a piece type 'uses up' a previous capture.
2. Once a piece type has been captured, it is remembered for the rest of the game.
3. When a piece captures another piece, it immediately promotes only to the piece type it captured (original contributor concept). (this is just piecePromotionOnCapture + 'using up' a capture, so not a separate option)

For 1 and 2, consider if there are any special rules if promotion would occur and no piece captured yet.

The pocket could be used to remember pieces, since you wouldn't want to combine this with drops. If they would be combined, consider if and how they would interact or not.

The promotionPieceTypes would list all possible promotions, then they would be unlocked by being in the pocket.

Expanded from a description of Parkros's variant in FSF Discord, #variant-configuration, 2024/04/16