Here's an idea for not so much a "Monster" Card, but a card that might show up if various political powers started getting sufficiently powerful/wealthy:
Elite Mercenary Band $0*
Action/Attack/Prize
Initially given to the player who gains the Xth victory point of the game.
+ 1 Action
All other players must discard down to 3 cards
After they do so, you will reveal all treasure cards in your hand.
If any other player has treasure cards in their hand that are worth more $ than the treasure cards you reveal,
they may discard those treasure cards to gain this card in their discard pile.
If nobody else gains this card, +1 Card, +$3
It makes things interesting, promotes big money a bit and provides some potential for someone who missed out the first time to steal the card from someone overconfident. I'm not sure if/how potions should be counted for this though...