OK, I see. This seems more about an issue how a mime piece that can be selected from a menu in a particular piece of software should behave to be most generally useful than a discussion about chess-variant design. In such configurable software the solution I discussed with Aurelian seems more flexible (and in fact upward compatible) than any of the options in the poll: allow the user to define an arbitrary mapping similar to the capture matrix between moved piece and what the mime should move as. The promoChoice strings could contain a newly defined special symbol to indicate "as the last-moved piece can promote". This decouples move mimicking from promotion mimicking.
OK, I see. This seems more about an issue how a mime piece that can be selected from a menu in a particular piece of software should behave to be most generally useful than a discussion about chess-variant design. In such configurable software the solution I discussed with Aurelian seems more flexible (and in fact upward compatible) than any of the options in the poll: allow the user to define an arbitrary mapping similar to the capture matrix between moved piece and what the mime should move as. The promoChoice strings could contain a newly defined special symbol to indicate "as the last-moved piece can promote". This decouples move mimicking from promotion mimicking.