There can be some reverse variations […] the loser is the one who loses any last remaining piece type.
This seems like an interesting idea, though at first glance it seems you might have to do something to incentivise captures as otherwise both sides would just avoid capturing in order to stop any of their own pieces becoming royal. Perhaps as a Jeu Forcé (either total or limited to e.g. pawns)? Either that or have slightly‐different armies so that there are some royal pieces from the beginning
This seems like an interesting idea, though at first glance it seems you might have to do something to incentivise captures as otherwise both sides would just avoid capturing in order to stop any of their own pieces becoming royal. Perhaps as a Jeu Forcé (either total or limited to e.g. pawns)? Either that or have slightly‐different armies so that there are some royal pieces from the beginning