5 second animated/video interstitials before the ranked battle and a question about the interstitial to start the fight would be a good way to make things tougher for bots and improve engagement.
Also, corporate sponsor promo cards. Limited edition rewards cards to promote a particular business or brand for a short period of time. Not everyone is going to be able to get these cards as they have a short print life, but that's the point when it comes to their value. Also, these cards would be forever a part of the game and will always be a way to get an ad impression through to the player or market participant. We all want that limited edition Ronald McDonald Life Leech summoner.