If you are going to mess with the trash, you have to realize that it's a shared resource pool, unlike the mats for NV, Pirate Ship and Island, but much more like the Trade Route mat.
What you don't want: Having to count cards in the trash pile during the game, or having different people count the same pile afterwards (because they don't trust each other or don't share the info). Also, for cards to interact with the trash, there needs to be something in the trash to begin with. This means that such a card must both be a trasher as well as a trash remover, but I think everyone got that idea.
You also have to be careful with shenanigans which allow very good trashed cards to be gained by the same player, possibly in the same turn.
This is why I think an Attack card can be nice here.
Rag and Bone Man - $5
Action - Attack
+$1
Choose one: Trash 2 cards from your hand; or every other player gains the top card from the Trash pile.
If the Trash pile is empty, they gain a Curse instead.
I found it difficult to make this very different from Ambassador. Ambassador lets you do both things at the same time: Get rid of your crap and pass it to others. This lets you do just one thing, but gives you a Coin to compensate. The Curse is an extra incentive and presents players with an interesting choice in the beginning: Trash your own crap or dish out Curses?
Be wary of reactions though:
- Watchtower lets a player move the gained card back (or the Curse to) the trash.
- Trader lets a player get a Silver instead, leaving the trash card on top for the next player (or Curse in the supply)
Giving it the right cost is always the hardest part. I think it's strictly better than Sea Hag, because trashed cards are generally just as bad as Curses and it gives players an option. That's why I settled on $5, up with Trading Post.
I have no real-world experience with designing cards, only theoretical, so this may be either worthless or overpowered, but it's just the thought of using the Trash as a source for attacks that I find interesting.