Some off-the-cuff ideas (haven't attempted to price the first three) related to Gold.
Prospector - Action
Reveal the top card of your deck. If it is
- a Gold, put it into your hand.
- a Prospector, discard it and gain a Gold in hand.
Otherwise, either discard the revealed card or put it back.
(They "pan for gold", so yeah.)
Gold Rush - Action
+1 Card
+1 Action
If you have three or more Gold Rushes in play, gain a Gold.
(An alternate take on the 'treasure map' idea; they're easier to line up and can be lined up repeatedly, but you need to line up more of them for a smaller reward.)
Con Artist - Action
+1 Action
Set aside a card from your hand face down. The opponent to your left chooses whether or not to reveal it.
If you reveal it: put it back into your hand. If it was a Gold, gain a Province.
Otherwise: +$3. At the beginning of your cleanup phase, discard the set-aside card.
("Play an arbitrary card as if it were a Gold", combined with a bluffing game. The reward for honesty needs to be pretty big, because it's basically a blank card whenever you're honest and don't get called. There needs to not be a real penalty for getting caught bluffing because it's often difficult or impossible to be honest.)
Archimedes - Action - $2
+1 Card
+1 Action
In games using this, add Electrum to the Supply.
Electrum - Treasure - $5
Worth $3 if you have an Archimedes in play, otherwise $2.
(Just playing with the story of weighing the King's crown. Notwithstanding the effects of having a cantrip Action in your deck, opening Archimedes/Electrum is strictly worse than if you could open Gold/nothing, but on weak boards it might be your best option on a 5/2 split, so I think these prices are okay.)