In my Coronation set
http://forum.dominionstrategy.com/index.php?topic=4663.0, I added this card:
Mayor
4
Action
Gain a Silver, placing it on top of your deck.
Look at the top 4 cards of your deck. Draw one, trash one, discard one, and place the other back on top.
Each other player chooses one for themself: +1 card, trash a card from their hand, gain a Silver.
It turned out very weak. I started with the opponent benefit part and based the rest on giving you all 3 parts.
Sheriff
5
Action
+1 Card
+1 Action
Gain a Silver, placing it in your hand. Trash a card from your hand.
Each other player chooses one for themself: +1 card, trash a card from their hand, gain a Silver.
This seems weak, too. I also thought of putting it on a Curser (inspired by andwilk's Town Fool):
Corrupt Sheriff
5
Action-Attack
+$3
Each other player gains a Curse, then chooses one for themself: +1 card, trash a card from their hand, gain a Silver.
What kind of card should get this kind of effect: A terminal or a non-terminal? Should it be on a $4, $5, $6? Should it be on an attack?