So we know that Dark Ages will have cards that do something when trashed. We also know that the official Dominion app has you actually gain cards from the trash when playing a Thief because of interactions with "future cards". Furthermore, Donald has confirmed that he will finally include the 'lose-track' rule in the rulebook of one of the remaining expansions.
All this leads me to believe that we'll see a card something like this:
Bad Penny
Types: Treasure
Cost: $5
Worth $2.
When you play this, [some interesting effect here].
When you trash this, put it [onto your deck/in your hand/in your discard pile].
