Renovate Action $4
You may trash a card from your hand. If it is an . . .
Action card: +1 Action
Treasure card: +$1
Victory card: +1 Card
You may gain a card costing up to $1 more than the trashed card.
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If you play this to turn your Estates into Silvers or to trash your Coppers, it's terminal. But you can use it as an Upgrade on Action cards later in the game, once you've cleaned out your junk.
This is a simple idea; it's probably already been done, although I haven't seen it. But if not, is this 1) worth trying; 2) undercosted; or 3) overpowered at $4 or underpowered at $5?
I was also thinking of trying this as a Transmute variant, in which you had to gain a card of a different type, but that would probably be not much better than Transmute and would be much more complex.