This should be straightforward, using the same technique as for widening cards_data[] and cards_ptr[].

The only wrinkles I foresee are that Deckdll.dll will have to support it too, and that the "You get a duplicate card of your choice!" interface might choke if there's too many choices.

Deckdll.dll will also have to enforce the 500-card limit in a deck; increasing that, or the limit of 200 different cards in a deck, is significantly harder. I know it does for Manalink, but it may not in the Shandalar codepaths.

I don't really expect balance issues with this, as I was fearful of earlier, considering some of the AI decks folks have been making.