I'm not getting why you would ever buy Banner. It does nothing for you at all on play, compared to Embargo's $2. And then, while Embargo can hurt or shut down an opponent's strategy if it's different from yours, Banner only affects the first time they purchase the card, and even then instead of giving them a Curse, it gives them a one-shot terminal.
Or you could put it on the pile you want to buy to make it only cost $4 instead of $5 or something, maybe that was the intent? Although then it's just a terminal Copper. I'm not sure when you would ever actually want to spent the money and action space to get one.
I'm going to take out the problematic Banner. Seeing that it's a variant on a principle that has come up more than once on this forum, that of putting a supply card onto a different pile, I thought it good to write my thoughts against it for any fan card creators who come to the same idea. With the introduction of split piles I anticipate this being more likely.
Banner was this:
Banner - Action Victory, $4 cost.
Return this to the supply. Move a Banner onto a different Supply pile that doesn't have a Victory on top. Cards on that pile cannot be gained or bought before it is.
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When you buy this, gain the card under it.
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1VP
It wouldn't just change 5-costs into 4, but any cost; Platinum, and even Fortune and the other Debt-cost cards could all be bought for $4, with the intention that the player would get a junk card with it for balance. With hindsight, Banner isn't really a junk card. You can use an action to remove it from your deck and put it on a pile irrelevant to the game, even if that's Curse, and you can then run the risk of giving the same power to your opponents.
Besides this, you could use it aggressively to try denying piles to your opponents, like those that cost cheaper or those that depend on being bought like Mint. This denies the pile to yourself too, which you may not always want to work around.
I'd like to use a card mocked up on these forums as a reference to what would happen with a card that makes large on this denying effect, being of less benefit to the buyer's deck:
Both Rabbits and Banner have in-built ways to still allow gaining cards from the piles they sit on. With a card that doesn't, a game with it and no extra buys would see any chosen pile completely denied, taken out the game. No player will want to use a precious buy and turn on that card just to open up the cards underneath, as it will not progress the deck's payload.
Rabbits' impact on a game will vary depending on the presence of other sources of +buy. If there are no others, the only way to gain a card from a pile it sits on, and avoid the total denial problem, is by playing another Rabbits. This boils down to two outcomes: either there are few Rabbits out because few are bought, and a player is likely denied a key card to their strategy because they don't have a Rabbits on a turn they can afford it; or there are many out because many are bought, and that leads seamlessly to my conclusion. And if there are other buys, Rabbits become more like attempts at junking the opponent, but that eventually come right back at you.
My conclusion with all such
kingdom cards that move onto other piles is this: they may add extra variety to games, but, with the greatest respect to kru5h and his Rabbits, I do not personally feel they are a welcome addition. They take away from all the central strategy elements of the game and instead make it more a mindless process of 'buy whatever's available and see what happens'.
And from there I find it's impossible to balance Banner. No cost is fair yet effective, for if it's too low it's too powerful at letting players buy whatever they want, if it's too high you can totally eliminate more piles, and if it's in the middle like at $4, as Gendo said it has little point.