More stealth and trickery cards would be pretty fun. I personally like the idea of a Buff card that makes the next card you play be hidden like a Trap card.
With it being a Buff card, it'd announce to the opponent that you are playing a card, but they won't exactly know what. I think this could certainly have some power behind it, so making it a rare and cost 3 points seems fair to me.
I like the idea of it since it seems good on just about any skill, like subtlety increasing your speed, going invisible for a bit, changing your weapon, or even sending a hot potato into their hand!