I had this funky idea for a card.
I don't really play with fan cards myself, but sometimes I get crazy ideas in my head and I need to get them out so that maybe some other players will like them and play with them.
I think the name Tax may have been used a couple of times before, but I can't come up with anything better.
Tax: $5
Treasure
$1
When you play this, choose one: +$1 for every card that has been gained by a player this turn; or gain a card costing up to $3.
Explanation
It doesn't matter who gained the cards. If you had played a Mountebank in a 4p game and dealt out 3 Curses and 3 Coppers, this card would be worth $7. Hell yeah, it scales hard in multiplayer games with Cursers, but I don't think that breaks the card necessarily; you still need to draw it in the same hand as the Mountebank if you want that combo and you need a +Buy to make full use of it. You could also for instance buy/gain a Cache first and with your regular buy/extra buy and have $3 more to spend. The card needs to self-synergize a bit to not make it worthless in games without gains or +Buys, that's why I added the Workshop-type action. I'm not too good with prices, so if this needs to be $6, it could be $6. It's just that a couple of alternative treasures are at the $5 point too.
Even if no one had gained anything, it would still be at least a Copper.
When you have both this and Horn of Plenty, you can just choose which you want to play first. If you pick HoP you get an extra $ for this card. Or you could gain a card costing more with HoP if you play this first.
I like having it as a Treasure instead of an action so you can always play it. That's why I made it gain only cards that are $3 or less where the cheap villages and cantrips are, so it couldn't be abused that hard.
What do you guys think?