If you are member of KIngdom or Secret Society, you can start or join card games in your group.
Gambling would be included in silver!
Also there could be arranged duels among Kingdoms or Kings.
I will tell you story of my real life country, Croatia. It is in Europe and it is older then 1000 years. We have a shield on our flag with 25 field which represents chess field. Chess game.
Legend of our country is that our Croatian King lead huge army against other nations and to not fight with armies, Kings of nations decided to play a game of chess. He who wins, gets are that is fought for.
Story says our King was very smart and he won chess game against other King and we got the land we live today, Croatia.
I always remember that story when I play Travian Card game.
Also, it would be nice feature to trade card games among players. I give you 1 NPC card you give me 1 crop Card.
I think there might be some legal issues if you allow underaged people to gamble against other players.
Trading i would personally like but i believe it would make the multiaccounting issues even worse.
Despite not liking arguments like "this supports forbidden methods very heavily" I have to bring it up right now. In Travian: Legends is the auction system a little bit different: The seller gets all the silver which is spend by the buyer of an item. There are a few bot networks who abuse this fact by spawning hundreds of accounts to grind silver (using adventures and selling items) and then selling this acquired silver automatically to real players in the gameworld by bidding a huge amount of silver on their items. The real players pay money for the silver, but (obviously) only a fraction of the official gold cost. I don't know the actual prizes, but maybe a few bucks for 70k silver (which would be 20 € if bought from TG). Yes, it's forbidden, yes, it's detectable, yes, some players get banned for it, but TG still loses a lot of money because of it probably - which is bad for all of us, who enjoy the game legally (well, bad for them who enjoy it illegaly aswell, I guess).
Being able to directly win or lose money would bring those bots to Kingdoms, hundreds of accounts with useless villages spawning on the map, I really don't want that.
Also legal issues, like Jallu mentioned.