Savings
A copper that, if not initially spent, is retained on your Reserve mat until such a time as you need it. I initially worded at beginning of your Buy phase, but I could probably loosen that. My wife and I tried it and often forgot to call it until other Treasures were already played. It never hurts to call it every Buy phase though, because if unspent it returns to the Tavern mat. Note that this cannot be called in the Action phase for, say, Black Market.
Planchet
A copper that can be made into any coin another player gained while this was in your hand. May alter the strategy of buying silver or not in the first couple turns. Do you risk giving the other players a free silver while retaining their buy to get another card? Do you trash this for silver and take the -$1 available for your own buy? Do you hang on to this in the hopes of turning it into a Gold, Platinum, or something else more useful down the line?
Heirlooms
Perhaps the most game-changing. It gives you a 9-card starting setup, which means only your first turn buy will make the first shuffle, and means you may potentially play an action card or a silver on turn 2. It gives you the possibility of hitting 6 on the first two turns. But this silver disappears if you do hit that critical 5 or 6 buy, so make it good. If you want to keep it around, it may incentivize multiple buys of $4 cards.
Barter
This one is a "worse than copper" coin. While in play, it is more expensive to buy Treasures. And so it incentivizes buying Actions or Victories-with-benefits.
Wage
A fairly fringe benefit on this. You may play it as an action to discard your deck. Like heirlooms, this can be used to get a first turn buy into circulation perhaps for turn 2. But you'll only be able to do this for a $4 or less, because it produces no $ when played as an Action.
Fiefdom
A Copper that leaves your deck when you play it. Sort of elf-trashing. But at a certain point they're quite valuable to retain, since two of them gives you a total of 4VP, 3 gives 6VP, and 4 would give 12VP. Of course, there's only one per player in the game. They go on top of Estate supply when played, and must be bought or gained before you can buy an Estate. You can still gain an estate as normal with cards that specifically call for them (Baron). Maybe with other gainers as well. I'm leaving that open for now.
