Rebuild
Type: Action
Cost: $5
+4 Cards. Discard 4 cards. Trash this card. Do this up to 4 times: Trash a card from your hand then gain a differently named card with the same cost as the trashed card.
So, the idea here is obviously a one-shot mass trash-for-benefit card. The +4/–4 cards is sifts cards into your hand that you want to trash. This could be used in a Curse-heavy game to transform up to 4 Curses into Coppers (and dead Curse-givers into more useful cards). It could be used late game to transform $5 cards into Duchies.
I'm wondering if it might be practical for mid-game deck transformation. For instance, you open with a bunch of Silvers. Then you buy the more expensive components for your engine ($5 drawing cards and Gold), then you use one of these to trash several Silvers for $3 Villages. Maybe that's impractical.
Anyhow, thoughts?