my entry:
Monkey's Paw
$4 - Action - Victory
Choose one: Gain a Curse and if you do, gain a Wish; or +$2 and you may have the player to your left gain this on top of their deck.
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Worth 1 VP if you gained this after your last turn of the game. Otherwise, worth -2 VP.
I think this card really hits this contest's theme well. But it does need some modification, in my opinion.
TLDR;
1 - make it cost $2 or cost $3 and gain card up to $6 instead of gain Wish.
2 - get rid of Victory type and VP on it, or make player to your left reveal their hand if you choose the option of giving it to them.
3 - assuming you keep Victory type, use existing Pass (to hand) mechanic instead of putting it on their deck, otherwise it should be Attack type as well.
It looks strictly worse than Leprechaun to me. Leprechaun gains a gold and (usually) receives a Hex (sometimes hurts a little, sometimes not). Monkey's Paw gains a card costing up to 6, but may not be in your deck until after an extra shuffle (if the Wish is drawn with terminal draw), and gains a Curse (junk in your deck and worth -1VP). Under the current design, I think it needs to cost $2, and even so may need to directly gain a card costing up to 6 instead of gaining a Wish (though that may allow it to be worth $3, comparable to Leprechaun).
Also, I think it needs to be an Attack type. I know Masquerade is not, but that gives you the option to Pass junk at the same time that you may receive junk. If the person on your left doesn't have a Cantrip in hand, and the game ends in the next round (but not on your current turn), then you just gave them -2VP. I think you can fix this (not have it be an Attack) by saying "you may Pass this to the player on your left" (i.e. gain to their hand). Then if you end the game this turn, they get +1VP. Otherwise, they at least have the option/choice to Pass it to the next person instead of keeping it for -2VP. This may mean playing it instead of another better terminal action. But like Masquerade, how bad that is depends on what you have in your hand.
There may also be a tracking/accountability issue, sort of. Generally, when the game ends, you would work through the last round of play to see how many Monkey's Paws each player recieved after their last turn and those are +1 whereas any others they have are -2VP
It gets really complicated though if you are going to end the game this turn (but people don't know it) and you have extra Actions and play a Masquerade after your Monkey's Paw (that you Pass to their hand under my change) and the player to your left has another Monkey's Paw in their hand already - they would obviously pass the one already worth -2VP, not the one currently worth 1VP, but when you end the game this turn and you all count points, they would have to prove that they kept the one worth 1VP (the next person would think they Passed it).
2 ways to fix it are to make the player to your left reveal their hand if you choose to Pass it to them, or get rid of the Victory type (and all VP on it) entirely, but I know you want to use this to raise the skill level required to use this card well (and it's pretty boring and too much like Leprechaun without the VP).
You may argue that this is a reason to put in on their deck instead of Pass it to their hand... but if you have a mega turn to end the game, we are right back to this problem if you play Monkey's Paw, Council Room, then Masquerade (could happen with Snowy Village, Champion, or just a well built engine with a lot of villages) - so it is a fringe case, but not that much of a fringe.