I'd love to see promo cards added, though most likely a patch will be needed should they go the route I'm thinking- which is making special promotional cards for special villagers cut from the games, such as Farley, Frillard, & Serena. However, there really aren't many cut special villagers, but they can make different cards unlock different clothing for each special villager, maybe even one that lets you put a QR on them? Or they could release some DLC sets that they don't plan on having return once they finish up the DLC. There's a lot of possibilities and I absolutely love the idea of promo cards extending the cards and adding content.
Of the current only 4 promo cards, 3 (Alternates of Stitches, Goldie, and Rosie) were/are packaged with Animal Crossing Happy Home Designer. The Isabelle variant promo card is a Japan exclusive, being also exclusive to one store brand in Japan.