If you were to allow yourself to publish one card (a promo, say) that for all sorts of reasons you would never, ever actually publish, what card would that be?
I can't really answer the question as given. I mean, it's not that I'm holding myself back. If there's a card I don't want published, I don't want it published. I allow myself, right now, and then, I pass, I don't want things published that I don't want published.
Sometimes a card is fun in playtesting but can't go out. It messes up the game in a fun way but it always does it and it's no good to always have that experience with the card. This is maybe the closest to what you're looking for. Man what was a good one of those. The ones I am thinking of are still just way too bad for me to get any joy thinking about them being published. Like War Axe was fun in its day; a Silver with a when-gain trashing attack. You have a sub-game of buying them out and then some players may be utterly out of the game but still trapped in it. It's not an experience I am looking to have more of or share, but I did enjoy those games.
Sometimes a card is too powerful in a way that doesn't let me just weaken it. Adventures had the TFB mix of +1's; we had a lot of fun with that crazy thing. You can't just always make the cards weak enough because of the coarseness of the costs and not wanting microtext.
Sometimes a great simple classic idea just doesn't happen to work in Dominion, like "each other player discards a card."
Sometimes a card looks cool on paper but fails to be fun. For example the card that increases numbers in another card's text, that sounds like a great promo until you try it.
Sometimes I like a card more than my playtesters, and end up abandoning it for that reason. Treasure Hunter is a good example, I mean that one did come out, but it was around for years, failing to make it into expansions, before it finally made it as one level on the Page path. If I'm forced to pick an outtake to be published, man, I take one of these. Maybe one of the Remodel your top card cards.