What about an Attack card that waits until a certain phase to deal its damage. Like an attack card that waits until the end of the action phase.
Assassin
5
Action - Attack
+4 Cards. Discard 2 cards or 1 copper.
At the end of your Action phase, each other player with 5 or more cards in hand reveals a number of cards equal to the number of Assassin cards you have in play. Each player discards one revealed card of your choice.
How would a Moat react to this? What if the player draws a Moat between the first and second time this is played in a turn?
Follow instructions in order, do all you can! So here's a potential Action phase:
I play Village and get +1 card, +2 actions.
I play Assassin and get +4 cards and discard a copper.
I play Governor and get +1 action, +3 cards. You get +1 card.
I play Assassin. You reveal Moat; I get +4 cards and discard a copper.
End of Action phase. The two Assassins activate simultaneously, so I choose the order.
First Assassin activates. You have 6 cards in hand and two Assassins are in play, so you reveal 2 cards and discard one.
Second Assassin activates. You have 5 cards in hand, but you Moated this Assassin, so it doesn't affect you.
I proceed to the Buy phase.