Okay, so there's Vineyard (Action-scaled Victory) and Silk Road (Victory-scaled Victory). Occasionally in this forum people suggest a Treasure-scaled Victory card, and the answer is usually that that wouldn't be interestingly different from a Province game, since getting as many Treasure cards as possible seems to enable Province acquisition pretty well already.
But: could a Copper-scaled Victory card be feasible?
There are two different ways to do it, I guess. One is to make it like Silk Road and Gardens, and be fairly easy to acquire with a deck full of Copper and green:
$4
Worth 1 VP for every 3 Coppers in your deck
...In which case you'd rush them while trying to get as much Copper as possible, not too different from the way you build a Gardens deck. This is probably overpowered since you can grow its value a lot faster than Gardens, although you have to sidetrack yourself from Copper-gaining in order to end the game on piles.
The other possibility strikes me as more interesting, but conceivably also broken in some way:
$6
Worth 1 VP for every 2 Coppers in your deck
This version introduces some tension: You want a lot of Copper in order to boost their value, but flooding your deck with Copper actually makes this card harder to buy (unlike the $4 version), so you have to thread the needle of boosting your Copper density, but not so fast as to make the $6 victory card unaffordable.
Thoughts?