Lots of ideas.
"When you gain this card, every other player gains a curse"
"When you gain this card, you may trash a card from your hand"
"When you gain this card, +1 VP token" (unlikely)
"WYGTC, you may put it on top of your deck"
"WYGTC, you may gain (a silver)(another copy of this card)(an estate)(a duchy)"
Reactions
"When you gain a card, you may reveal this from your hand. If you do, set this aside on the table. At the beginning of your next hand +2 cards and discard this card." (or many similar things) - incidentally, this could be your first just reaction that's not also an action. Conceivably you could also have victory-reactions or treasure-reactions.
"When an opponent plays an attack card, you may reveal this from your hand. If you do, gain a card costing up to $4 and discard this card"
I'm thinking these cards may have pretty big impacts on the very first two turns. I mean, whatever you do on turn one might actually impact turn 2. That's pretty crazy.
I'm actually most intrigued by the 3 victory cards. That's quite a few, and victory and this theme don't obviously go together to me.