- an ELO rating system
- maybe a story mode with quests incorporated in a similar way to the Puzzle Quest
- gain XP when winning hands/games/ranking up, give bonuses for the XP in the form of character items, pet dragons, whatever
- dueling between players, betting who wins a game of poker or a game of a coin toss
I like this first idea - elo is huge in MMOs and if that was added to gambling it'll entice a lot more people and make sure that everyone wants to keep playing for extra elo.
I think the biggest aspect you'd want to focus would be PVP fights or flips. Your able to take a direct cut out of it, and they are a lot more fun then just betting against the house - being able to fight someone means your able to basically duel anyone and having fights where damage is RNG would be cool.