This is an interesting idea. There could be a cap on the number of effects people can activate to keep things from going crazy. Like you start with 3 moves and get 2 every turn or something. People could also use custom cards to help prop up a favorite card or archetype that would otherwise be lackluster.

Since its a yugioh rp I imagine it'll have cheesy scenarios and the cast making dumb remarks as they observe their teammates duels? The duelest kingdom arc would be a fun place to start.