Coppermaster seems quite crazy, I think. +4 or more coins (occasionally less) and +buy make it quite fearsome.
What if the bonus weren't stackable on multiple plays? It would let one buff more freely the playability of the card (making it non-terminal, cantrip, offering +buy, +cards or similar). It would be less crazy when it gets crazy, but more often useful.
i'm thinking of a flat increase like "+2 Actions, Copper is worth 2" (or a treasure worth 1-2 with the same bonus)
Another cool thing to try is having the bonus scale on something else, like the value of the best treasure you have in play, or the number of Silvers you've played this turn, or the number of different Treasures you have in play...
That way you need to play only one card to get the bonus, leaving you free to try to build an engine focused on maxing the value of coppers and more likely to have place for the +buys and stuff. (I doubt my examples are good ideas, since they seem broken as engine payload, and weak otherwise)
Since there's no stopping the flow of bad ideas from me, here's another one:
+2 cards
Trash a card from your hand.
At the start of your buy phase, reveal your hand. Coppers you play this turn are worth a number of coins equal to the number of Estates in your hand.
BLAM! Megaturn!