MArcomage

Make your hand hidden until your next turn.

You can only cards. You can't specify how long will they reamin hidden, since continuous effects are not supported.

Ok change it to
"Make your hand hidden"
but I think that would not be strange in a non-hidden game, can the game type allow the trigger of another effect?

Yes, you could make an if-then-else statement based on game mode.

Ok

I change it then to
>In hidden games: Make your hand hidden
>In no-hidden game: replace your Common cards with Fog, Uncommon with Unsual Fog and Rare with Strange Fog
(Those fogs cards are quick cards that "hides" a C/U/R card)

(If in non-hidden-games to hide the hand if thought too strange)

Now that i'm thinking this way maybe only the non-hidden game effect would be enough (also for hidden games)