I thought of a card today that has both an interesting level of choice and could really give a boost to certain copper-centric strategies.
Investment
$5
+1 action
Trash a copper. If you do, choose one: Gain a silver, putting it into your hand; or gain three coppers, putting them into your hand.
On one hand, it's a weak mine in it's trashing ability, but it has the advantage of +1 action, so it can self-synergize a bit without becoming broken. It also presents the choice of getting +$2 now but weakening your deck or +$1 and strengthening it. Finally, it could really give a boost to copper-centric strategies like counting house and coppersmith. It might even be a bit too strong with coppersmith, but that does require you to have both cards obtained already and only gives four more treasure than the coppersmith would have given originally (if played just once).
I haven't playtested this at all but am very curious to hear your thoughts on it.