Mendicant. First of all I must say I love copper strategies. But I’m not too big on gaining estates. Why don’t you make it a duchy instead. And make it cost 5. So it’s a little harder to get. And make it so you can only get one duchy per buy phase. Cause throne rooming or playing more than one might be chaotic it might get you a bunch of victory points but it would also ruin your deck too soon. Especially if it was multiple estates.
Here’s another idea.
Instead of a simple gain. Why don’t you make it like this.
+1 Card
+1 Action
Gain a copper to your hand.
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At the end of your buy phase if you have no other treasure besides copper in play. You may trash a victory card from your hand. And then gain a duchy.
This way you could still keep it at 4$ and you wouldn’t be able to gain to many duchies at once. Also you would have to buy extra estates if you wanted more than 3 duchies. And you wouldn’t probably get 3 duchies in a turn even if you 3 mendicants in play. Also it wouldn’t be quite like rebuild cause you could never go above a duchy. And also you could make it a little more interesting. Instead of saying duchy you could say a victory costing up to 6$. That would make it more useful if you were playing with other expansions. Like fairgrounds, Duke, Silk Road, tunnel, farmland would be epic!, the castles up to haunted castle, distant lands, island, nobles, harem, mill, feodum, gardens, Shepard and cemetary. And still keep out province, colony, some of the castles, and the one of kind vineyard.