As pointed out before, I think they would have to be 'ruined' versions of cards in the base set.
Ruined market - we know that one, +1 buy.
I think Ruined Smithy and Ruined Village are a good bet. Either +1 or +2 cards or actions, respectively, not sure which. +1 if they're meant to be entirely useless, +2 if they're meant to be potentially useful but way too weak to buy. (Despite the fact that ruined market gives +1 buy, that's actually a playable card - in engines, I wouldn't mind getting one if it was the only +buy source, so it's better than a +1 card or +1 action ruin.)
+1 coin is probably a good bet too, I don't know the name for it though. Ruined woodcutter? A terminal copper is pretty bad.
Ruined chapel - trash one card from your hand - also seems plausible. It might be too good for a ruin, given that when you get ruins getting a free trasher (even a weak one) is probably fine, like picking up a transmute in a curse game. But maybe a ruined remodel: trash one card from your hand, gain another card costing the same as the trashed card? Would be useful for turning ruins into coppers, at least, or trading a ruin you don't want for a ruin you do. Useful, but not so useful as to be worth having in your deck intentionally.