While this is in play, you may not gain a copy of (this card name here)
So what happens if one of these is in play, and somebody Swindles a card into it?
Anything else, or sometimes nothing to replace the swindled card !
I don't think it's clear. If one card (Swindler) says to gain something, and another card (yours) says not to, it's not clear which rule should take precedence. There's nothing anywhere that says the player of Swindler can't choose your card, nor anything that says the card must be trashed upon being gained to avoid being gained. It's simply an unplayable situation, similar to the problem fan cards with "you may only play one of these per turn" clauses have with Golem.
But there's an easy solution. Change the word "gain" to "buy," and you're all set. Other cards never force you to buy other cards, and Grand Market and Contraband are precedence for restrictions on what you're allowed to buy. True, you might still be able to finagle a way to gain them (Horn of Plenty, University, etc), but if you're clever enough to manage that, I think you ought to be able to. (Nothing's cooler than Smuggling a Grand Market from someone who has worked hard to be able to buy one.)
Anyway, my take on the card is that it's a cool idea but way too swingy based on what else is in the kingdom. A dead card often (no villages or other durations), too weak to bother with usually (villages but no other durations), and a powerhouse occasionally (non-terminal duration cards present). The hugeness of the range of possible benefits remind me of Donald X's problem with his original draft of Duke ("Worth 1 VP for every Duke in your deck"). At its weakest, it's a terminal Caravan (undesirable even at $2); at the next level up, it's a buyless Wharf (still strong at $5); above that, it's out of control. Those are huge jumps.
The scaling issue might be solvable by using a threshold mechanic instead of a scaling one, which is how Conspirator and Fool's Gold works. Maybe something like "If you have 2 or more duration cards in play (including this), then now and on your next turn, +2 Cards." Not necessarily those exact numbers, but that structure.
However, while that (maybe) solves the problem of it becoming too strong in some kingdoms, it doesn't do anything about it being a dead card in others. It needs to work without outside support. How do we do that? Make this conditionally a non-terminal. Something like "If you have no other duration cards in play, +1 Card, +1 Action." That's not a sufficient solution, because it still only lets us play two per turn without outside help, and the benefit is too small for it to be worth trying to collide two copies together. If we consider this problem in light of the suggestion to make the drawing benefit next-turn-only, we chance upon a convenient solution:
+1 Card, +1 Action. If you have at least two Duration cards in play (including this), then at the start of your next turn, +2 Cards.
While this is in play, you may not buy a copy of this card.Now we can spam multiples now, then play a huge hand later. When you collide two in your hand, you get a next-turn Wharf. When you collide three, you get almost a next-turn Tactician. This is still pretty strong, but the buy restriction imposes a penalty on anyone seeking to rush the pile, as in order to buy another copy, you have to refrain from playing the ones in your hand. This penalty is actually a bigger deal in this version of the card than in the original, because this version would otherwise be free to play; with the original, maybe you just play a different terminal.
How does this play when there is outside support? Well, if any other durations are present, you can try to line them up and thereby not have to waste the first one. (Somewhat akin to how, in Conspirator games, sometimes you have to get the chain started with one Village and one unenabled Conspirator -- it's fine, but you'd rather have had that first one enabled.) So it's a good thing, but not game-breakingly stronger.
More significantly, the presence of other duration cards double your chances of colliding this card with another duration card. Because you don't just have to draw it in hand with another duration card. You can draw it in the hand
after you play another duration card. (Note that unenabled copies of this card don't get left out until the next turn.) So that adds to the power level. But still perhaps not gamebreakingly strong.
Now, how to price it? I think it would be weak for $5, but I'd be nervous about it at $4 except that the buy restriction makes it harder to accrue multiples. Because of that, I actually wonder if $3 would be fine. You don't have to worry too much about $3 making them easier to pick up with extra buys, just because of the buy restriction. And opening with two of this card doesn't seem gamebreakingly strong. So I'm going to guess $3 is okay. However, that's a very sketchy guess. It's hard for me to envision how this would play in a real game.
Anyway, that's my take. It may be that this version of the card loses what intrigued you about your draft, in which case, oh well, back to the drawing board. But hopefully my ruminating will help you refine it.
