Alternately, could you have 'Trash a card from your hand. Gain a different card of the same cost'.
This lets you turn your Curses and Ruins into Coppers, which is mildly helpful, I suppose. It also acts as a friendly Swindler, which transforms useless Sea Hags and Witches, combos with fortress, etc. Or is this too powerful? Should the above limit it to cards of cost $0?
The "Different" prevents you from running out the Provinces.