The idea came to me when playing with Band Of Misfits that I would want a reverse of the card. That instead of being a specific card once played, it would be a 'counts as that card' while still in hand before being played.
The type would necessarily have to be action-attack-reaction-treasure-victory. Leaving aside the problem of how to get this all printed at the bottom of a card, it still leaves the issue of curse and duration card types. Duration I don't think would be a problem as there are no cards that directly interact with type=duration. Curse I would not want to have as a listed type because of all the problems associated with 'gain a curse' wording. Gaining a curse should only be gaining a curse, not gaining your choice of curse or joker. There are at least three cards this would be an issue on (that I know of, could be more). Fortune Teller, Torturer, and Mountebank. Fortune Teller is ok since the card is of type victory anyway. Torturer and Mountebank can be made to interact correctly by making the reaction part read 'if another player plays an attack card, you may discard this card. If you do, you are unaffected by that attack.
The next hurdle to get past is the problem of being both action and treasure. But I think this can be overcome by careful selection of the action wording. 'When you play this, do X' would work when it was played either as an action or as a treasure. Much like Horn Of Plenty. And like Horn Of Plenty the value would need to be zero to avoid undue complications of it being played in the action phase.
Now to the heart of the issue. What action would it be? Going with the Joker theme, I like +1 card best. It actually 'becomes' another card. Since it is treasure, it probably should generate income. So +1$ as well. But make that an explicit vanilla bonus (like on contraband) rather than just making it a treasure worth 1. Since it is a victory card, 1VP should be there too. So now it is beginning to look like a half-Harem. (I thought for a bit that it should be +1 coin token +1 victory chip, but I think this would make it too powerful.)
For the attack, I would want something amazingly mild. We dont want to actually use this as an attack. We just want to give it enough to make it into type=attack. A silly idea to cover this would be "all other players revel a card from their hand". Any defense card used to counter it would be redundant.
Its cost I would make 3. Low enough that you can sometimes pick it up with an extra buy, but it would still interact with any attack card that trashes between 3-6.
With all that in place, how would it interact? Mostly I think its use would be to load up with specific draw cards, then hold them until your buy phase when you can play them as a weaker Venture. It would combo this way with Scout, with Crossroads, and with Vagrant (And to a lesser extent, Wandering Minstrel). It would also do this with Sage but might not be as good a combo. Transmute, Ironworks, Tribute, and Iron monger would combo a bit TOO well though. In fitting with the Joker theme it should probably be "pick-1" but I dont know how to word that. It helps Venture, Silk Road, Conspirator, Vineyard, Bank, Menagerie, Harvest, Horn of Plenty, Scheme, Urchin, Death Cart, Journeyman, and Herald. Though not strongly in any of them. Interactions with Peddler would be strange.
Overall, I wish I could word it so that it would do stuff like interact with stuff like Wishing Well, Baron, Copper-smith, Duke, Treasure Map, Explorer, Apothecary, Counting House, Tournament, Young Witch, Feodum, Rats(!), or Mystic. But I think doing so would make it more overpowered than what I would want for a low value card.