Here is my entry. I can see getting this early and trashing all Coppers with it, then if you can't trash it use it to just filter your Victory cards.
Secret Market
$3 Action
+1 Card
+1 Action
You may discard a card for +1 Card.
You may trash a Treasure for +1 Buy and +$1.
I like this but I think it's too strong for $3. Maybe even could cost $5, but probably okay at $4. Two of these could get rid of all your coppers quick and be a market+a little sifting in the meantime. If there's very little to no other +buy, I can see buying Coppers to keep these working.
<Hillside Village>
Honestly, I'm surprised this hasn't been done before, considering Plaza is a thing.
Actually, I'm pretty sure I have seen that done before probably more than once with other people's fan cards, but that doesn't mean it's a bad idea. If you wanted to, you could make it only +1 Action by default +the Action from the played treasure still makes it a Village. Then you could make it cheap. (The game could use more decent $2 villages IMO.)
I've also used a fanmade card very similar to X-tra's Sanctum (except it discards a victory card instead of playing an estate for the same effect). It's one of my favorite fanmade Villages.
Now to think of my own submission...