czzzz's Grimoire
At first glance, this doesn't seem very powerful. When you trash Estates, you get a delayed +1 Card, and this acts like a triggered Goatherd. Non-terminal trashers are always good though (Monastery). And the Shadowing from trash is brilliant. Trashing Coppers with this gives you +1 Coffers essentially. It looks very fun and balanced.
Hidden Cave • $4 • Action - Night
Shadow a Worker or Action from play or the Supply.
Worker: Stone Cutter
Stone Cutter • $4 • Worker
Trash a Hand card to gain either: a differently named card costing up to $1 +$1 per empty supply pile more; or a card costing up to $3.
-
This costs $1 more if two Supply piles are empty.
BryGuy's Hidden Cave/Stone Cutter
Okay, so I'll ignore the Worker for now. Without that, this turns into a weaker Throne Room. Part of what makes TR a great card is that you can stack them. That's impossible with Hidden Cave. Also, there's an infinite loop for cards like this: Play Hidden Cave, Shadow a HC from the Supply, use it to replay Hidden Cave, shadowing another HC, replaying HC... With Training/Travelling Fair this gives you inifinite coins and buys. You could easily fix this by making HC pure Night, and I dislike how it's an Action - Night without that being the whole card idea like Werewolf. HC+Bridge looks fun. And Shadowing Workers from play is a great idea after they're tapped. Stone Cutter looks a bit needlessly complicated; I would remove the up to $3 option.
Winner: czzzz's Grimoire
Runner up: BryGuy's Hidden Cave/Stone Cutter
Honorable mentions: my alarm that didn't go off, that Dominion spinoff that DXV never made, and my metal doorknob that's made of wood.