I would imagine that there will be a card that can gain from the trash, and it could also be a card that could Upgrade itself (or is tied to such a card, a la Young Witch and its bane). Upgrading would be "trash this, gain that (not in the supply)." And then you could use a "gain from trash" to re-gain an upgradable card. Maybe a card could upgrade into one of multiple things, so you would actually NEED to gain it from the trash in order to get more copies of something else.
You could care about the trash as an attack. "Choose a card in the trash. Each opponent gets a copy of it from the supply." Or you can just force each opponent to gain a card from the trash.
There could be a Forge-like card that combines cost of X items in the trash -- think of it as making the most of the scraps available, as you might have to do in hard times.
I figure Barbarians and Plague are going to be new attack cards. Maybe Beggar, also.
This is exciting.