Splinterlands has so many value pools (cards, cards levels, power, dec, potions, drop rates, lands, airdrop, sps, credits, guild stats, rewards chests, rank, etc etc) that specialty cards could impact any of these areas and be a really interesting addition to the game. I like the idea of animated foil cards to work like quest potions, for each one you have it gives you an additional potion for quests up to a certain amount, or you can delegate the foil card to a certain area to increase a certain resource you want to gain.